Add comprehensive upload/download API tests and error handling tests

- Add upload API tests: upload without tag, artifact creation, S3 storage
- Add download tests: tag: prefix, Content-Type/Length/Disposition headers
- Add download tests: 404 for nonexistent project/package/artifact
- Add checksum header tests: ETag, X-Checksum-SHA256
- Add error handling tests: timeout behavior, checksum validation
- Add resource cleanup tests: verify no orphans on failed uploads
- Add graceful error response tests: JSON format, detail messages

CHANGELOG.md

@@ -14,6 +14,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added `@pytest.mark.large` and `@pytest.mark.concurrent` test markers
 - Added `generate_content()` and `generate_content_with_hash()` test helpers
 - Added `sized_content` fixture for generating test content of specific sizes
+- Added upload API tests: upload without tag, artifact creation verification, S3 object creation
+- Added download API tests: tag: prefix resolution, 404 for nonexistent project/package/artifact
+- Added download header tests: Content-Type, Content-Length, Content-Disposition, ETag, X-Checksum-SHA256
+- Added error handling tests: timeout behavior, checksum validation, resource cleanup, graceful error responses
 - Added production deployment job triggered by semantic version tags (v1.0.0) with manual approval gate (#63)
 - Added production Helm values file with persistence enabled (20Gi PostgreSQL, 100Gi MinIO) (#63)
 - Added integration tests for production deployment (#63)