FEATURE: WarmPool API for server

[Pangjiping]

Summary

• refactor batchsandbox pool service
• create_pool_service() in factory.py; warm_pool_service from it in lifecycle (dedicated K8sClient, not shared with sandbox).
• WarmPool API only when kubernetes + workload_provider → batchsandbox; otherwise 501 with clearer messaging.
• Pool routes: path validation, 422 docs, drop unused X-Request-ID params; Pool schemas use ConfigDict.
• PoolService: stricter CRD mapping, list 404 → 503 unchanged, ApiException → HTTP status + apiserver message via kubernetes_api_exception.py; internal cleanup / less duplication.

Testing

• Not run (explain why)
• Unit tests
• Integration tests
• e2e / manual verification

Breaking Changes

• None
• Yes (describe impact and migration path)

Checklist

• Linked Issue or clearly described motivation
• Added/updated docs (if needed)
• Added/updated tests (if needed)
• Security impact considered
• Backward compatibility considered