Make LSP backpressure cancellation test deterministic

[Widthdom]

Summary

• Stage the LSP cancellation frame until the active symbol request and backpressured response write are both observed.
• Hold the symbol request at a cancellation-aware synchronization boundary instead of a fixed five-second delay.
• Document the deterministic test contract in both English and Japanese and add the issue changelog fragment.

Root cause

The active workspace-symbol test hook and queued cancellation each relied on the same fixed timing window. Under full-suite load, the hook could finish before the cancellation was processed, producing a successful response where the test expected a cancellation error.

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false
• Targeted Release test on net8.0 and net9.0
• Targeted Release net8.0 test repeated in 20 independent processes
• Full Release net8.0 suite: 10,233 passed, 7 skipped, 0 failed
• Full LspServerTests suite during Codex adversarial review: 103 passed on each of net8.0 and net9.0
• Codex adversarial review: no blocking/actionable issues found

Documentation and changelog

• Updated both language sections of TESTING_GUIDE.md.
• Added changelog.d/unreleased/4785.internal.md.

Follow-up candidates

None.

Fixes #4785