test: added upload validation test cases

Description: This MR adds comprehensive unit tests for backend content and file validation schemas. Reviewed app/schemas/upload_validation.py and identified key validation models and helper functions (ContentValidationBase, file validators, chunked uploads, file hashes, record updates, upload finalization). Created tests/unit/schemas/test_upload_validation.py with focused Pydantic validation tests for these schemas and helpers. Added positive and negative cases covering critical boundaries

*Closes #111 (closed)