Add integration tests for XAML Language Server client/extension wiring

Summary

The XAML Language Server feature has good unit test coverage for individual providers (parser, diagnostics, completions, formatting, hover, definition), but lacks integration-level tests for the extension/client wiring layer.

What's missing

The following flows have no automated coverage:

• activateXamlLanguageServer / deactivateXamlLanguageServer lifecycle
• winapp.xaml.restartLanguageServer command registration and execution
• Config enable/disable toggle (winapp.xaml.languageServer.enable) triggering client start/stop
• Startup failure recovery path
• Client lifecycle serialization under concurrent config changes

Why it matters

All provider-level unit tests can pass while activation, restart, and config flows are broken, since those tests exercise the providers in isolation without the VS Code extension host.

Suggested approach

Add a focused VS Code API-stub test suite (or extension-host integration test) that covers:

1. Activate → client starts successfully
2. Config disable → client stops
3. Config re-enable → client restarts
4. Restart command → stop + start cycle
5. Deactivate → clean shutdown

Context

Surfaced during PR review of the XAML Language Server feature (round 6, finding M2).