Fix IndexWatchRunner cancellation test race

[Widthdom]

Summary

• Synchronize IndexWatchRunnerTests cancellation checks on the actual watch-start output instead of a fixed sleep.
• Always cancel/drain the dedicated watch task before restoring redirected console writers.
• Document the watch-loop test synchronization rule in TESTING_GUIDE.md.
• Keep the Windows/net9.0 CI lane bounded by skipping coverage on that slowest matrix leg and not retrying TestSessionTimeout aborts.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Debug -p:UseSharedCompilation=false --filter "FullyQualifiedName~IndexWatchRunnerTests"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Debug --no-restore --filter "FullyQualifiedName~CiWorkflowTests|FullyQualifiedName~IndexWatchRunnerTests"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Debug --no-restore --filter "FullyQualifiedName~CiWorkflowTests"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --framework net9.0 --no-restore --filter "FullyQualifiedName~CiWorkflowTests|FullyQualifiedName~IndexWatchRunnerTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet test CodeIndex.sln -c Debug --no-build -p:UseSharedCompilation=false
• PowerShell probe: Tee-Object preserves $LASTEXITCODE for success and failure paths.
• Codex adversarial review: No blocking/actionable issues found.

Documentation / Changelog

• Updated TESTING_GUIDE.md.
• Added changelog.d/unreleased/3343.fixed.md.
• Added changelog.d/unreleased/3358.fixed.md.

Follow-up Candidates

None.

Fixes #3343
Fixes #3358