Fix ci-bonsai-daily: configure unmerged_blobs mock in git_mergetool tests (#8574)

test_returns_none_when_report_file_absent/empty build a MagicMock repo
without configuring index.unmerged_blobs(), so it returned a truthy
MagicMock and git_mergetool's load-bearing "unresolved conflicts remain"
fallback (tool/ifcgit.py:646-647) returned that list instead of None -
failing "assert [] is None". The production fallback is correct and
intentionally left untouched; the tests just misrepresented the
"mergetool resolved cleanly" scenario they are named for. Set
mock_repo.index.unmerged_blobs.return_value = {} in both.

Verified in headless Blender: test/tool/test_ifcgit.py::TestGitMergetool
2 failed / 1 passed -> 3 passed.

This change was made with the assistance of an AI tool.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>