Comprehensive test coverage for _get_conversation_mtime() and its
integration with _collect_sessions().
Test cases:
- Claude sessions: file exists, file missing, OSError on stat,
missing project_dir, missing session_id
- Codex sessions: transcript_path exists, transcript_path missing,
discovery fallback, discovery returns None, OSError on stat
- Edge cases: unknown agent type, missing agent key
- Integration: mtime included when file exists, omitted when missing
- Change detection: mtime changes trigger payload hash changes
Uses mock patching to isolate file system interactions and test
error handling paths without requiring actual conversation files.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
