Fix commit-diff timeout regression test

[Widthdom]

Summary

• make the shared hanging fake Git return the commit expected by rev-parse --verify <ref>^{commit}
• keep the timeout and cancellation paths reaching diff-tree while preserving production single-commit validation
• document the regression contract in English and Japanese

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net8.0 --filter "FullyQualifiedName~GetChangedFilesFromCommit_FailsWhenGitCommandTimesOut|FullyQualifiedName~GetChangedFilesFromCommit_CancelDuringGitCommand_ThrowsOperationCanceled" --no-build --no-restore -p:UseSharedCompilation=false (2 passed)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net8.0 --filter FullyQualifiedName~GitHelperTests --no-build --no-restore -p:UseSharedCompilation=false (65 passed)
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false (0 warnings, 0 errors)
• dotnet test CodeIndex.sln -c Release --no-build --no-restore -p:UseSharedCompilation=false (net8.0: 11,749 passed / 7 skipped; net9.0: 11,270 passed / 423 skipped)
• dotnet run --project tools/CodeIndex.Changelog -- check (42 fragments validated)
• root and workspace index health checks passed

Documentation and changelog

• updated the English and Japanese test guidance in TESTING_GUIDE.md
• added changelog.d/unreleased/5130.fixed.md

Review

• Codex adversarial review: No blocking/actionable issues found.

Follow-up candidates

• None.

Fixes #5130