Git timeout regression test cannot reach diff-tree after single-commit validation

[Widthdom]

Summary

GitHelperTests.GetChangedFilesFromCommit_FailsWhenGitCommandTimesOut consistently fails on current origin/main under net8.0. The fake git used by the test returns a fixed hash for rev-parse --verify, while ValidateSingleCommitRef now requires the resolved hash to match the requested temporary commit. Validation therefore throws ArgumentException before the fake diff-tree process can exercise the intended timeout path.

Reproduction

dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net8.0 --filter FullyQualifiedName~GetChangedFilesFromCommit_FailsWhenGitCommandTimesOut --no-build --no-restore

Expected: the test reaches diff-tree and observes the bounded timeout as InvalidOperationException.

Actual: Invalid commit ID ... Git could not resolve it to a single commit is raised as ArgumentException.

Acceptance criteria

• The fake git validation response remains consistent with the temporary commit passed by the test.
• The test reaches the diff-tree timeout path and verifies that the child process is reaped.
• Production single-commit validation remains strict.