Validate ignored_referencing_fields configuration and add tests

[SupraSummus]

Summary

This PR adds validation to the garbage collection command to ensure that configured ignored_referencing_fields actually match existing ForeignKey fields. It also includes comprehensive tests and documentation updates.

Key Changes

• Configuration Validation: The find_fk_fields() function now tracks which ignored field paths are actually matched against existing ForeignKeys. If any configured paths don't match, a CommandError is raised with a clear message about the stale or incorrect configuration.
• Test Coverage: Added two new test cases:
  • test_ignored_referencing_field_does_not_keep_row_alive(): Verifies that ignored referencing fields don't prevent garbage collection
  • test_unmatched_ignored_field_path_fails_loudly(): Ensures the command fails with a helpful error message when configuration contains invalid field paths
• Model Updates: Added Cache and CacheLog test models to support the new test cases
• Documentation: Updated README to clarify that invalid field paths in gc_ignored_referencing_fields will cause the command to fail, catching configuration errors early

Implementation Details

• The validation happens in find_fk_fields() by maintaining a matched_ignored set that tracks which ignored paths were successfully matched
• Unmatched paths are detected and reported before the function returns, ensuring errors are caught even in dry-run mode
• The error message is descriptive and suggests the issue is either stale configuration or a typo

https://claude.ai/code/session_01NWap56ibZN8sey8mQfa8uw