test: add FileResult, DownloadFileResult, UploadFileResult, and import_with_fallback tests

Increases express.py test coverage from 56% to 79%. New tests cover:
- FileResult open/close/read/write, context manager, delete_after_use
- DownloadFileResult URL parsing, domain/target_dir env var resolution, caching
- UploadFileResult validation (missing file, directory, namespace, domain mismatch)
- import_with_fallback success paths and error cases
- DataModel construction

Author: LukasGold