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