Comprehensive testing of parallel commit message generation

[oleander]

Task

Create comprehensive test coverage for the parallel git diff analysis implementation.

Requirements

Happy Path Testing

• Single file commits
• Multi-file commits (2-5 files)
• Large commits (dozens of files)
• Various diff types:
  • File additions
  • File deletions
  • File modifications
  • File renames

Error Condition Testing

• Individual file analysis failures
• API failures and network issues
• Rate limiting scenarios
• Invalid diff formats
• Missing configuration

Performance Testing

• Verify performance targets:
  • Single file: ~4 seconds
  • Five files: ~4.3 seconds vs ~16s sequential
• Measure under normal network conditions
• Test graceful degradation with network issues/API slowdowns

Integration Testing

• Verify respects user configuration settings
• Test with various API timeout values
• Test with different max message length settings
• Verify debug output contains correct timing information

Success Criteria

• All tests pass consistently
• Performance improvements validated
• Error handling verified as graceful and informative
• Works correctly with all supported diff formats
• Configuration settings properly respected

Part of #81