IntelliSense doesn't work with external projects in file-based apps

Type: Bug

## Issue Description ##
In C# 2.140.8 and later, IntelliSense is broken for file-based apps with external projects. Any class referenced from such projects has no documentation or autocompletion. This feature worked properly in C# 2.130.5.

## Steps to Reproduce ##
1. Create a file-based app.
2. Import an external project, for example `#:project Nixill@0.15.0`.
3. Use code from that project, for example `using Nixill.Collections;` and `AVLTreeSet set = [];`
4. Hover over those imported classes or start typing a method on that variable.

## Expected Behavior ##
Documentation and autocompletion pop-ups appear.

## Actual Behavior ##
Documentation and autocompletion pop-ups do not appear.

Extension version: 2.142.16
VS Code version: Code - Insiders 1.124.0-insider (0035c783eccf8a2efbdb972dfe62becb51f11fc6, 2026-06-05T15:12:36+02:00)
OS version: Windows_NT x64 10.0.19045
Modes:

### Logs ###

### C# log ###

```
2026-06-05 17:19:04.696 [info] Locating .NET runtime version 10.0.5
2026-06-05 17:19:05.692 [info] Dotnet path: c:\Users\Nixill\AppData\Roaming\Code - Insiders\User\globalStorage\ms-dotnettools.vscode-dotnet-runtime\.dotnet\10.0.8~x64~aspnetcore\dotnet.exe
2026-06-05 17:19:05.692 [info] Activating C# + C# Dev Kit...
2026-06-05 17:19:05.708 [info] AI features are disabled (chat.disableAIFeatures). Skipping Roslyn Copilot component.
2026-06-05 17:19:06.328 [info] info: Program[0]
2026-06-05 17:19:06.328 [info] Server started with process ID 35828
2026-06-05 17:19:07.563 [info] [Program] Language server initialized
2026-06-05 17:19:07.564 [info] [Program] Monitoring client process 7436 for exit
2026-06-05 17:19:08.331 [info] [initialized] [Microsoft.CodeAnalysis.LanguageServer.FileBasedPrograms.FileBasedProgramsEntryPointDiscovery] Finished discovery in 'c:\Users\Nixill\Documents\Git\APManual-ETS2' in 27 milliseconds
2026-06-05 17:19:09.736 [info] [textDocument/_vs_getProjectContexts] [Microsoft.CodeAnalysis.MSBuild.BuildHostProcessManager] .NET BuildHost started from c:\Users\Nixill\.vscode-insiders\extensions\ms-dotnettools.csharp-2.142.16-win32-x64\.roslyn\Microsoft.CodeAnalysis.LanguageServer.exe reloading to start from C:\Program Files\dotnet\dotnet.exe to match necessary SDK location.
2026-06-05 17:19:11.641 [warning] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Project C:\Users\Nixill\AppData\Local\Temp\roslyn-canonical-misc\8e4e84db-fe6a-4f54-8589-e9c22353f5a2\Canonical.csproj has unresolved dependencies
2026-06-05 17:19:11.885 [info] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Successfully completed load of c:\Users\Nixill\Documents\Git\APManual-ETS2\scripts\CompanyLocations.cs
2026-06-05 17:19:11.889 [warning] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Project C:\Users\Nixill\AppData\Local\Temp\roslyn-canonical-misc\8e4e84db-fe6a-4f54-8589-e9c22353f5a2\Canonical.csproj has unresolved dependencies
2026-06-05 17:19:11.909 [info] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Successfully completed load of c:\Users\Nixill\Documents\Git\APManual-ETS2\scripts\MakeJson.cs
2026-06-05 17:19:11.939 [info] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Restoring Canonical.csproj: Running dotnet restore on C:\Users\Nixill\AppData\Local\Temp\roslyn-canonical-misc\8e4e84db-fe6a-4f54-8589-e9c22353f5a2\Canonical.csproj
2026-06-05 17:19:11.940 [info] [textDocument/_vs_getProjectContexts] [Microsoft.CodeAnalysis.LanguageServer.DotnetCliHelper] Using dotnet executable configured on the PATH
2026-06-05 17:19:14.187 [info] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Restoring Canonical.csproj: Determining projects to restore...
2026-06-05 17:19:15.022 [info] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Restoring Canonical.csproj: Restored C:\Users\Nixill\AppData\Local\Temp\roslyn-canonical-misc\8e4e84db-fe6a-4f54-8589-e9c22353f5a2\Canonical.csproj (in 176 ms).
2026-06-05 17:19:19.933 [info] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Restoring Canonical.csproj: Running dotnet restore on C:\Users\Nixill\AppData\Local\Temp\roslyn-canonical-misc\8e4e84db-fe6a-4f54-8589-e9c22353f5a2\Canonical.csproj
2026-06-05 17:19:20.773 [info] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Restoring Canonical.csproj: Determining projects to restore...
2026-06-05 17:19:21.028 [info] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Restoring Canonical.csproj: All projects are up-to-date for restore.
2026-06-05 17:19:25.363 [info] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Restore complete
2026-06-05 17:19:25.363 [info] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Completed (re)load of all projects in 00:00:16.4512059
2026-06-05 17:19:25.485 [info] [workspace/didChangeWatchedFiles] [LanguageServerProjectLoader] Successfully completed load of c:\Users\Nixill\Documents\Git\APManual-ETS2\scripts\CompanyLocations.cs
2026-06-05 17:19:25.500 [info] [workspace/didChangeWatchedFiles] [LanguageServerProjectLoader] Successfully completed load of c:\Users\Nixill\Documents\Git\APManual-ETS2\scripts\MakeJson.cs
2026-06-05 17:19:25.500 [info] [workspace/didChangeWatchedFiles] [LanguageServerProjectLoader] Completed (re)load of all projects in 00:00:00.0225535
2026-06-05 17:19:42.222 [info] [workspace/didChangeWatchedFiles] [LanguageServerProjectLoader] Successfully completed load of c:\Users\Nixill\Documents\Git\APManual-ETS2\scripts\MakeJson.cs
2026-06-05 17:19:42.222 [info] [workspace/didChangeWatchedFiles] [LanguageServerProjectLoader] Completed (re)load of all projects in 00:00:00.0106179

```

### C# LSP Trace Logs log ###

```

```

## Environment Information ##
**VSCode version**: 1.124.0-insider
**C# Extension**: 2.142.16
**Using OmniSharp**: false

Dotnet Information

Host:
Version: 10.0.8
Architecture: x64
Commit: 94ea82652c
RID: win-x64

.NET SDKs installed:
No SDKs were found.

.NET runtimes installed:
Microsoft.AspNetCore.App 10.0.8 [c:\Users\Nixill\AppData\Roaming\Code - Insiders\User\globalStorage\ms-dotnettools.vscode-dotnet-runtime\.dotnet\10.0.8~x64~aspnetcore\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 10.0.8 [c:\Users\Nixill\AppData\Roaming\Code - Insiders\User\globalStorage\ms-dotnettools.vscode-dotnet-runtime\.dotnet\10.0.8~x64~aspnetcore\shared\Microsoft.NETCore.App]

Other architectures found:
None

Environment variables:
DOTNET_CLI_UI_LANGUAGE [en-US]
DOTNET_NOLOGO [true]

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download

Visual Studio Code Extensions

|Extension|Author|Version|Folder Name|
|---|---|---|---|
|asl-extension|B0sh|1.1.0|b0sh.asl-extension-1.1.0|
|codespaces|GitHub|1.18.13|github.codespaces-1.18.13|
|craftos-pc|JackMacWindows|1.2.3|jackmacwindows.craftos-pc-1.2.3|
|csdevkit|ms-dotnettools|3.20.199|ms-dotnettools.csdevkit-3.20.199-win32-x64|
|csharp|ms-dotnettools|2.142.16|ms-dotnettools.csharp-2.142.16-win32-x64|
|datapack-language-server|SPGoding|4.8.3|spgoding.datapack-language-server-4.8.3|
|debugpy|ms-python|2026.6.0|ms-python.debugpy-2026.6.0-win32-x64|
|delete-file-and-keep-open|davidalejandroaguilar|1.0.2|davidalejandroaguilar.delete-file-and-keep-open-1.0.2|
|EditorConfig|EditorConfig|0.18.2|editorconfig.editorconfig-0.18.2|
|even-better-toml|tamasfe|0.21.2|tamasfe.even-better-toml-0.21.2|
|fix-json|oliversturm|0.2.0|oliversturm.fix-json-0.2.0|
|geo-data-viewer|RandomFractalsInc|2.6.0|randomfractalsinc.geo-data-viewer-2.6.0|
|gitignore|codezombiech|0.10.0|codezombiech.gitignore-0.10.0|
|gitlens|eamodio|2026.6.51206|eamodio.gitlens-2026.6.51206|
|hexeditor|ms-vscode|1.11.1|ms-vscode.hexeditor-1.11.1|
|isort|ms-python|2026.6.0|ms-python.isort-2026.6.0|
|java|redhat|1.54.0|redhat.java-1.54.0-win32-x64|
|jupyter|ms-toolsai|2026.6.2026060301|ms-toolsai.jupyter-2026.6.2026060301-win32-x64|
|jupyter-keymap|ms-toolsai|1.1.2|ms-toolsai.jupyter-keymap-1.1.2|
|jupyter-renderers|ms-toolsai|1.3.0|ms-toolsai.jupyter-renderers-1.3.0|
|lua|sumneko|3.18.2|sumneko.lua-3.18.2-win32-x64|
|markdown-all-in-one|yzhang|3.6.3|yzhang.markdown-all-in-one-3.6.3|
|markdown-preview-enhanced|shd101wyy|0.8.28|shd101wyy.markdown-preview-enhanced-0.8.28|
|nugetpackagemanagergui|aliasadidev|2.1.1|aliasadidev.nugetpackagemanagergui-2.1.1|
|open-in-browser|techer|2.0.0|techer.open-in-browser-2.0.0|
|python|ms-python|2026.4.0|ms-python.python-2026.4.0-win32-x64|
|rainbow-csv|mechatroner|3.24.1|mechatroner.rainbow-csv-3.24.1|
|rainbow-tags|voldemortensen|0.4.1|voldemortensen.rainbow-tags-0.4.1|
|remote-explorer|ms-vscode|0.5.0|ms-vscode.remote-explorer-0.5.0|
|remote-ssh|ms-vscode-remote|0.124.0|ms-vscode-remote.remote-ssh-0.124.0|
|remote-ssh-edit|ms-vscode-remote|0.87.0|ms-vscode-remote.remote-ssh-edit-0.87.0|
|remote-wsl|ms-vscode-remote|0.104.3|ms-vscode-remote.remote-wsl-0.104.3|
|scs-script|Frankity|0.0.6|frankity.scs-script-0.0.6|
|se-csproj-extensions|selcukermaya|0.0.7|selcukermaya.se-csproj-extensions-0.0.7|
|shopify-liquid|sissel|4.0.1|sissel.shopify-liquid-4.0.1|
|syntax-mcfunction|MinecraftCommands|1.0.1|minecraftcommands.syntax-mcfunction-1.0.1|
|vscode-autohotkey-debug|zero-plusplus|1.11.1|zero-plusplus.vscode-autohotkey-debug-1.11.1|
|vscode-autohotkey-plus-plus|mark-wiemer|6.7.3|mark-wiemer.vscode-autohotkey-plus-plus-6.7.3|
|vscode-autohotkey2-lsp|thqby|3.0.9|thqby.vscode-autohotkey2-lsp-3.0.9|
|vscode-avalonia|AvaloniaTeam|12.2.0|avaloniateam.vscode-avalonia-12.2.0|
|vscode-computercraft|JackMacWindows|1.1.1|jackmacwindows.vscode-computercraft-1.1.1|
|vscode-dotnet-runtime|ms-dotnettools|3.0.2|ms-dotnettools.vscode-dotnet-runtime-3.0.2|
|vscode-firefox-debug|firefox-devtools|2.15.0|firefox-devtools.vscode-firefox-debug-2.15.0|
|vscode-gcode-syntax|appliedengdesign|0.7.7|appliedengdesign.vscode-gcode-syntax-0.7.7|
|vscode-github-actions|github|0.31.5|github.vscode-github-actions-0.31.5|
|vscode-gradle|vscjava|3.17.3|vscjava.vscode-gradle-3.17.3|
|vscode-java-debug|vscjava|0.59.0|vscjava.vscode-java-debug-0.59.0|
|vscode-java-dependency|vscjava|0.27.5|vscjava.vscode-java-dependency-0.27.5|
|vscode-java-pack|vscjava|0.31.0|vscjava.vscode-java-pack-0.31.0|
|vscode-java-test|vscjava|0.45.0|vscjava.vscode-java-test-0.45.0|
|vscode-json5|mrmlnc|1.0.0|mrmlnc.vscode-json5-1.0.0|
|vscode-jupyter-cell-tags|ms-toolsai|0.1.9|ms-toolsai.vscode-jupyter-cell-tags-0.1.9|
|vscode-jupyter-slideshow|ms-toolsai|0.1.6|ms-toolsai.vscode-jupyter-slideshow-0.1.6|
|vscode-maven|vscjava|0.45.3|vscjava.vscode-maven-0.45.3|
|vscode-nuget-package-manager|jmrog|1.1.6|jmrog.vscode-nuget-package-manager-1.1.6|
|vscode-pylance|ms-python|2026.2.1|ms-python.vscode-pylance-2026.2.1|
|vscode-python-envs|ms-python|1.30.0|ms-python.vscode-python-envs-1.30.0-win32-x64|
|vscode-sqlite|alexcvzz|0.14.1|alexcvzz.vscode-sqlite-0.14.1|
|vscode-xml|redhat|0.29.2026051209|redhat.vscode-xml-0.29.2026051209-win32-x64|
|vscode-zipexplorer|slevesque|0.3.1|slevesque.vscode-zipexplorer-0.3.1|
|vsliveshare|ms-vsliveshare|1.1.122|ms-vsliveshare.vsliveshare-1.1.122|
|wordcount|ms-vscode|0.1.0|ms-vscode.wordcount-0.1.0|;

C# Settings

|Setting|Value|
|---|---|
|preferCSharpExtension|false|
|compilerDiagnosticScope|fullSolution|
|analyzerDiagnosticScope|fullSolution|
|enableXamlTools|true|
|useServerGC|true|;

System Info

|Item|Value|
|---|---|
|CPUs|11th Gen Intel(R) Core(TM) i5-11600KF @ 3.90GHz (12 x 3912)|
|GPU Status|2d_canvas: enabled
GPU0: VENDOR= 0x10de, DEVICE=0x1b82 [NVIDIA GeForce GTX 1070 Ti], DRIVER_VENDOR=NVIDIA, DRIVER_VERSION=32.0.15.6094 *ACTIVE*
GPU1: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.19041.5794
Machine model name:
Machine model version:
direct_rendering_display_compositor: disabled_off_ok
gpu_compositing: enabled
multiple_raster_threads: enabled_on
opengl: enabled_on
rasterization: enabled
raw_draw: disabled_off_ok
skia_graphite: disabled_off
trees_in_viz: disabled_off
video_decode: enabled
video_encode: enabled
webgl: enabled
webgpu: enabled
webnn: disabled_off|
|Load (avg)|undefined|
|Memory (System)|15.89GB (2.84GB free)|
|Process Argv|--crash-reporter-id eaa957b1-6421-4350-86e9-44de8f4ee528|
|Screen Reader|no|
|VM|0%|

A/B Experiments

```
vsliv368cf:30146710
pythonvspyt551cf:31249598
nativeloc1:31118317
dwcopilot:31158714
dwoutputs:31242946
copilot_t_ci:31333650
g012b348:31231168
pythonrdcb7:31268811
pythonpcpt1:31399616
6518g693:31302842
63221493:31336333
envsactivate1:31349248
editstats-enabled:31346256
cloudbuttont:31366566
3efgi100_wstrepl:31403338
cp_cls_t_966_ss:31526232
4je02754:31455664
c3h7c220:31478652
cp_cls_t_1081:31454832
conptydll_true:31485575
ia-use-proxy-models-svc:31446143
e9c30283:31453065
test_treatment2:31471001
46204921:31447328
idci7584:31454084
e3e4d672:31454087
ei9d7968:31462942
f860i809:31458078
7e884298:31462391
7e187181:31482583
githubstableflight:31469463
h08i8180_copy:31475366
ec1eg703:31520046
ddid_t:31478204
hmra_i5g22:31518061
getcmakediagnosticsoff:31489825
61f49681:31505879
7df3h592:31491241
cp_cls_t_1082:31516087
logging_enabled_new:31490725
jb_cp_cls_c_632:31524806
32d76977:31503652
ha629193:31508444
cdk-lw-on:31524445
31fi7170_t:31517328
a1ije391_t:31520709
hgf2d445:31510900
prpt_srch:31513639
05755904:31517511
jf4hg949:31526829

```