[Bug]: C# Sidecar Initialization Fails Permanently When Opening a Projectless Directory

What happened?

Description

When opening a projectless directory in VS Code (like an empty folder or the Desktop) and subsequently creating/opening a .cs file, SharpLsp fails to provide any language features (autocomplete, diagnostics, etc.).

Root Cause

When a folder is opened in VS Code, the Rust host eagerly sends a workspace/open request to the sidecars with the directory path. The C# sidecar attempts to find a .sln or .csproj. Upon finding nothing, it delegates to OpenProjectlessAsync(path) in an attempt to load it as a single-file app.

However, because the path is a directory and not a .cs file, OpenProjectlessAsync correctly rejects it and returns a Failure. Because the workspace/open initialization failed, the Rust host skips starting the health monitor for the sidecar, and the C# sidecar gets stuck in a permanent uninitialized state (_solution = null).

Any .cs files opened afterwards have their edits stashed infinitely in UpdateDocumentTextAsync, rendering the extension broken for that session.

Solution

This can be fixed cleanly inside the C# sidecar without requiring changes to the Rust host's eager-loading logic:

1. Defer Initialization on Directories: In WorkspaceManager.OpenCoreAsync, if target is null but the path is a directory, set a flag (_isProjectlessDirectory = true) and return Success. This allows the Rust host to successfully boot the sidecar and start the health monitor.
2. Lazy Load on First File: In WorkspaceManager.UpdateDocumentTextAsync, check if _isProjectlessDirectory is true. If it is, intercept the first .cs file opened, clear the flag, and invoke OpenProjectlessAsync(filePath) with the precise file path before acquiring the _solutionMutationLock.

This restores the single-file mode logic exactly as originally designed, building the ad-hoc workspace around the .cs file as soon as the user opens it.

Steps to reproduce

1. Install the SharpLsp extension in VS Code.
2. Go to File -> Open Folder... and select a completely empty directory (e.g., a new folder on your Desktop) that does not contain any .csproj or .sln files.
3. Once the folder is open, create a new file named test.cs.
4. Type some C# code into the file (e.g., Console.WriteLine();).
5. Expected behavior: The extension provides autocomplete, IntelliSense, and diagnostics for the single file.
6. Actual behavior: No suggestions or diagnostics appear, and the extension fails to parse the file because the sidecar initialization has died silently in the background.

Component

C# sidecar (Roslyn)

Language

C#

SharpLsp version

0.16.0

Editor & OS

VS Code, Windows

Relevant logs

Start in WorkspaceManager.OpenCoreAsync and UpdateDocumentTextAsync, then reproduce by opening an empty VS Code directory and creating a .cs file. Trace the projectless-directory state and the first document update. Done means the C# sidecar initializes successfully, loads the file lazily, and provides autocomplete and diagnostics.