test: add comprehensive upload functionality tests

- Add TestArtifactUploader_SuccessfulUpload for normal upload flow validation
- Add TestArtifactUploader_MultipleArtifacts for batch upload scenarios
- Add TestArtifactUploader_ValidatesInputs for input validation edge cases
- Add TestArtifactUploader_HandlesLargeFiles for large file upload testing
- Add TestArtifactUploader_NetworkFailure for network timeout simulation
- Add TestArtifactUploader_PartialUploadFailure for mixed success/failure scenarios
- Add TestArtifactUploader_PermissionDenied for access control testing
- Add TestArtifactUploader_ContextCancellation for context cancellation handling
- Add TestArtifactUploader_InvalidArtifactPath for file system error scenarios
- Add TestArtifactUploader_ConcurrentUploads for thread safety validation

Includes comprehensive mock infrastructure with configurable failure scenarios,
realistic error types, and concurrent access safety. Tests cover upload
reliability, error handling, retry logic, and performance with large files.

Co-authored-by: Ona <no-reply@ona.com>

Author: leodido