test(git): add tests for get_filenames_in_commit with git_reference

[HankyStyle]

Description

Add missing test coverage for the get_filenames_in_commit function.

This PR adds:

• test_get_filenames_in_commit: happy path test without git_reference, verifying correct filenames are returned from the last commit
• test_get_filenames_in_commit_with_git_reference: happy path test with a specific commit SHA as git_reference, verifying only files from that specific commit are returned

Closes #1860

Checklist

• I have read the contributing guidelines

Was generative AI tooling used to co-author this PR?

• Yes
  Generated-by: Claude Opus 4.6 + Antigravity

Code Changes

• Add test cases to all the changes you introduce
• Run uv run poe all locally to ensure this change passes linter check and tests
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes

Documentation Changes

• Run uv run poe doc locally to ensure the documentation pages renders correctly
• Check and fix any broken links (internal or external)

Expected Behavior

Running uv run pytest tests/test_git.py -k "get_filenames_in_commit" should pass all 3 tests:

1. test_get_filenames_in_commit — returns correct filename from the last commit
2. test_get_filenames_in_commit_with_git_reference — returns only files from the specified commit SHA, not from other commits
3. test_get_filenames_in_commit_error — raises GitCommandError when git command fails (existing test, unchanged)

Steps to Test This Pull Request

1. Checkout the branch
2. Run the specific tests:

   uv run pytest tests/test_git.py::test_get_filenames_in_commit tests/test_git.py::test_get_filenames_in_commit_with_git_reference tests/test_git.py::test_get_filenames_in_commit_error -v
   

3. All 3 tests should pass

Additional Context

Related issue: #1860

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.99%. Comparing base (4fbeae7) to head (926911d).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1890   +/-   ##
=======================================
  Coverage   97.99%   97.99%           
=======================================
  Files          60       60           
  Lines        2689     2689           
=======================================
  Hits         2635     2635           
  Misses         54       54           

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[Lee-W]

should it be

Suggested change

	assert filename in filenames
	assert [filename] == filenames

instead?

[bearomorphism]

+1

and probably add another commit with more files to validate the behavior that the function only resolves filenames from the last commit