Chat Sessions panel: multi-select + Delete only removes one session instead of all selected

- Copilot Chat Extension Version: 0.58.0
- VS Code Version: 1.130.0 (e5381e4f30c68abae911cad10f9d02e93c427d61, 2026-07-21)
- OS Version: macOS 26.5.2 (Darwin x64 25.5.0)
- Feature: Agent mode (affects all modes — this is a Chat Sessions panel UI issue)
- Selected model: Claude Opus 4.6 (model-independent — bug is in the Sessions panel tree view)
- Logs: N/A (UI behavior, no errors in console)

Steps to Reproduce:

1. Open the Chat Sessions panel in the sidebar
2. Click on a session entry
3. Shift+Click on another session entry several positions away (multiple sessions highlight as selected)
4. Right-click the selection → choose "Delete"
5. **Expected:** All selected sessions are deleted
6. **Actual:** Only one session is deleted (typically the first or last in the selection); the rest remain

## Impact

Users accumulating many chat sessions have no efficient way to prune them. The only alternatives are deleting one-by-one (tedious) or using "Clear All Workspace Chat History" (too aggressive). A working multi-select delete would fill the gap between these extremes.

## Additional Environment Details

- Electron: 42.6.0
- Chromium: 148.0.7778.280
- Node.js: 24.18.0
- V8: 14.8.178.38-electron.0