Chat history is missing from the VS Code UI although local chat session files still exist

Type: Bug

### Title
Chat history is missing from the VS Code UI although local chat session files still exist

### Description
My previous Copilot Chat conversations no longer appear anywhere in the VS Code Chat UI.

There is no chat-history entry in the Chat view menu, and the Command Palette does not offer a command such as "Chat: Open Chat History" or "Chat: Show Chats".

I ran `Developer: Reload Window`, but the history remains unavailable.

### Expected behaviour
Previously saved Copilot Chat conversations should be listed in the Chat UI and be reopenable.

### Actual behaviour
The conversation list is empty/unavailable in the UI, even though the underlying local session files are still present.

### Evidence
The following local folder still contains chat sessions:

`%APPDATA%\Code\User\workspaceStorage\\chatSessions`

In my case:
- 23 `.jsonl` session files exist in `chatSessions`
- 16 complete transcript files exist in:
`%APPDATA%\Code\User\workspaceStorage\\GitHub.copilot-chat\transcripts`
- The oldest available transcript is from July 2026, and newer ones exist from today.
- `chat.sessionSync.enabled` is set to `true`.
- The synchronized session index appears empty, while local transcript files remain intact.

### Environment
- VS Code version: 1.135.0
- OS: Windows
- Copilot Chat history synchronization: enabled

### Steps to reproduce
1. Use Copilot Chat in a workspace and create several chat sessions.
2. Restart/reload VS Code.
3. Open the Chat view.
4. Observe that no prior chats/history entry is available, despite the local `.jsonl` sessions and transcripts still existing.

### Additional request
Please provide a supported way to rebuild the UI session index from existing local `chatSessions` / transcript files, or restore the chat-history UI when the index becomes detached from those files.

VS Code version: Code 1.135.0 (08d4889f9ec4a1685d257b9b95de036c8e1ce1e5, 2026-08-25T14:26:52Z)
OS version: Windows_NT x64 10.0.26200
Modes:

System Info

|Item|Value|
|---|---|
|CPUs|12th Gen Intel(R) Core(TM) i5-1245U (12 x 2496)|
|GPU Status|2d_canvas: enabled
GPU0: VENDOR= 0x8086, DEVICE=0x46a8 [Intel(R) Iris(R) Xe Graphics], DRIVER_VENDOR=Intel, DRIVER_VERSION=32.0.101.7085 *ACTIVE*
GPU1: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.26100.8875
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)|15.64GB (3.41GB free)|
|Process Argv|--crash-reporter-id 14a2106b-4381-4143-9b02-00c50fac083a|
|Screen Reader|no|
|VM|0%|
Extensions (21)

Name|Identifier|Author|Version
---|---|---|---
Go to Character Position|davidfreer.go-to-character-position|davidfreer|2.1.0
GitLens — Git supercharged|eamodio.gitlens|eamodio|19.0.1
GitHub Copilot Chat Exporter|fengzehan.vscode-copilot-exporter|fengzehan|0.1.1
Go|golang.go|golang|0.56.1
Jasmine Test Explorer|hbenl.vscode-jasmine-test-adapter|hbenl|1.8.3
Test Explorer UI|hbenl.vscode-test-explorer|hbenl|2.22.1
Marp for VS Code|marp-team.marp-vscode|marp-team|3.6.1
Python Debugger|ms-python.debugpy|ms-python|2026.6.0
Python|ms-python.python|ms-python|2026.4.0
Pylance|ms-python.vscode-pylance|ms-python|2026.3.1
Python Environments|ms-python.vscode-python-envs|ms-python|1.36.0
Remote - SSH|ms-vscode-remote.remote-ssh|ms-vscode-remote|0.128.0
Remote - SSH: Editing Configuration Files|ms-vscode-remote.remote-ssh-edit|ms-vscode-remote|0.87.0
WSL|ms-vscode-remote.remote-wsl|ms-vscode-remote|0.104.3
Remote Development|ms-vscode-remote.vscode-remote-extensionpack|ms-vscode-remote|0.26.0
Makefile Tools|ms-vscode.makefile-tools|ms-vscode|0.12.17
Remote Explorer|ms-vscode.remote-explorer|ms-vscode|0.5.0
Remote - Tunnels|ms-vscode.remote-server|ms-vscode|1.5.3
Test Adapter Converter|ms-vscode.test-adapter-converter|ms-vscode|0.2.1
Live Server|ritwickdey.liveserver|ritwickdey|5.7.10
Veracode Scan for VS Code|veracode.veracode-vscode-plugin|Veracode|1.16.3

A/B Experiments

```
vsliv368:30146709
binariesv615:30325510
nativeloc1:31344060
dwcopilot:31170013
dwoutputs:31242946
copilot_t_ci:31333650
e5gg6876:31282496
pythonrdcb7:31342333
6518g693:31463988
aj953862:31281341
envsactivate1:31551504
cloudbuttont:31379625
3efgi100_wstrepl:31403338
ec5jj548:31422691
diffpatch-lysithea-24-production:31574143
cp_cls_t_966_ss:31526232
4je02754:31466945
c3h7c220:31478652
ge8j1254_inline_auto_hint_haiku:31490510
cp_cls_t_1081:31454832
conptydll_true:31498968
e9c30283:31461165
46204921:31447328
capico_a_098a3278:31555650
ei9d7968:31496641
nes-extended-on:31455476
chat:31457767
8hig5102:31480529
89g7j272:31518289
i2gc6536:31499202
52612955:31516516
ddid_t:31478206
hmra_i5g22:31518061
ja75b849:31575055
7df3h592:31512476
cp_cls_t_1082:31535311
logging_enabled_new:31498466
j0d79568:31499440
jb_cp_cls_t_632:31543129
56dj4588:31512888
32d76977:31512328
ha629193:31508444
jh5f2457_c:31540921
jbcp_cls_pctr_t:31531130
61138546:31518536
cp_intellij_t_nes:31548657
d7b18187:31526828
ahp-both-windows:31556933
sessions_3:31532893
ihg5j128:31534457
f412h606:31551416
9gg7f176:31542112
enable_editor_pane_layout:31569726
allow-none:31555437
36h42362:31564511
c7c27ce7:31554789
1h923230:31564177
1532g621_copy:31554320
treatment-23-1:31555779
unuse_dynamic_mcp:31555281
0d8dfbc3:31570291
switchtoauto:31559321
0c1h4866:31566224
vrbsty_tr:31561058
session-archive:31558132
5b8j3302:31564601
mangle-name-control:31572633
9e5id457:31573761
39a5d156:31574986

```