[Phase 2.1] Implement OSS/S3 backend using object_store

[zhexuany]

Summary

Implement the Storage trait for Alibaba OSS using the object_store crate with S3-compatible API.

Parent Epic

• [Epic] Distributed Roboflow with Alibaba Cloud (OSS + ACK) #9 Distributed Roboflow with Alibaba Cloud

Dependencies

• Depends on: [Phase 1.5] Create StorageFactory for backend instantiation #25 (StorageFactory)

Tasks

1. Create src/storage/oss.rs
2. Define OssStorage struct with:
   • store: Arc<dyn ObjectStore> from object_store crate
   • runtime: tokio::runtime::Runtime for blocking operations
   • config: OssConfig for settings
3. Define OssConfig struct with:
   • endpoint: String
   • bucket: String
   • access_key_id: String
   • access_key_secret: String
   • region: String
   • use_internal_endpoint: bool
4. Implement OssStorage::new(config: OssConfig):
   • Create tokio runtime
   • Configure AmazonS3Builder with OSS settings
   • Set force_path_style for OSS compatibility
   • Build object_store client
5. Implement Storage trait for OssStorage:
   • reader(): Use get() with runtime.block_on(), wrap body in adapter
   • writer(): Create OssWriter that buffers and uploads on drop/close
   • exists(): Use head() operation
   • size(): Use head() operation
   • metadata(): Use head() and convert
   • list(): Use list() with pagination handling
   • delete(): Use delete() operation
   • copy(): Use copy() operation
6. Create OssWriter struct implementing Write:
   • Buffer writes to internal Vec or temp file
   • Upload on flush() or drop()
   • Handle large writes with multipart (delegate to [Phase 2.2] Implement multipart upload for large files #12)
7. Handle S3 eventual consistency considerations
8. Add connection pooling configuration
9. Feature-gate entire module under cloud-storage
10. Write integration tests with MinIO or localstack
11. Document OSS-specific configuration

Acceptance Criteria

• OssStorage implements all Storage methods
• Works with Alibaba OSS endpoints
• Works with AWS S3 (for testing with MinIO)
• Internal tokio runtime handles async operations
• Connection pooling configured
• All integration tests pass
• Feature-gated under cloud-storage

Files to Create

• src/storage/oss.rs

Configuration

Setting	Description	Default
endpoint	OSS endpoint URL	Required
bucket	Bucket name	Required
access_key_id	Access key	Required
access_key_secret	Secret key	Required
region	Region code	cn-hangzhou
use_internal_endpoint	Use internal network	false