- Implement unit tests to cover `Image` creation, metadata management, saving/loading, and error handling.
- Add separate constructors for `Full` and `Partial` image types (`new_full`, `new_partial`).
- Introduce robust path handling methods (`metadata_dir`, `image_json_path`) for consistent metadata structure.
- Enhance error diagnostics with `InvalidPath` variant in `ImageError`.
- Update `Cargo.toml` for test module inclusion.
