Add request-path coverage for completion during XAML project reload

Is your feature request related to a problem? Please describe.

The XAML language server preserves the last full project context so completion remains available while a replacement MSBuild/Roslyn context is loading. Current tests cover stale-context storage, retirement after failed loads, and completion stage behavior separately, but they do not send an actual textDocument/completion request while replacement loading is held in flight. A future wiring regression could stop passing allowStale: true without those focused tests failing.

Describe the solution you'd like

Add a deterministic server-level test harness that can hold project-context replacement loading pending, issue textDocument/completion, and verify that project completions come from the stale full context until replacement succeeds or fails. Verify that non-completion semantic requests do not consume stale context and that stale completion is retired after a null or faulted replacement.

Additional context

Identified during the PR #50 battle-test hardening review. Existing coverage is in AsyncSingleFlightCacheTests.cs and CompletionProjectStageTests.cs; the missing layer is request-path wiring through XamlLanguageServer.CompleteAsync.

Read AsyncSingleFlightCacheTests.cs and CompletionProjectStageTests.cs, then trace request-path wiring through XamlLanguageServer.CompleteAsync. Build the deterministic server-level harness described in the issue and verify completion uses the stale full context during replacement loading, semantic requests do not, and stale completion is retired after null or faulted replacement.