Stabilize ancestor ignore watcher coverage on macOS

[Widthdom]

Summary

• Make the ancestor-ignore watch integration test invoke the production enqueue callback after writing the real ancestor .gitignore on macOS.
• Keep actual FileSystemWatcher delivery coverage on Linux and Windows while retaining the bounded database reconciliation assertion on every platform.
• Document the platform-specific test contract in both the English and Japanese sections of TESTING_GUIDE.md.
• Add the bilingual changelog fragment changelog.d/unreleased/4966.internal.md.

Root cause and scope

On macOS, FileSystemWatcher/FSEvents delivery for a newly written ancestor .gitignore can be silently delayed or dropped. The reported net9.0 test timed out, and an isolated net8.0 run reproduced the same timeout when real delivery was preserved. Injecting the exact production enqueue callback showed that classification, debounce, and full reconciliation complete within the existing bound.

This PR changes test orchestration only; production watcher behavior is unchanged. The deterministic handoff is limited to macOS, where runtime delivery is not a stable test oracle.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net9.0 --no-restore --no-build -p:UseSharedCompilation=false --filter FullyQualifiedName=CodeIndex.Tests.IndexWatchRunnerTests.RunCore_SubprojectObservesAncestorIgnoreFileChanges_Issue4592
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net8.0 --no-restore --no-build -p:UseSharedCompilation=false --filter FullyQualifiedName=CodeIndex.Tests.IndexWatchRunnerTests.RunCore_SubprojectObservesAncestorIgnoreFileChanges_Issue4592
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net9.0 --no-restore --no-build -p:UseSharedCompilation=false --filter FullyQualifiedName~CodeIndex.Tests.IndexWatchRunnerTests (51 passed)
• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: No blocking/actionable issues found.

Follow-up candidates

None.

Fixes #4966