fix(k8s): Kubernetes executor pod cleanup

[justin-tahara]

Summary

• isolate Kubernetes websocket exec calls from pod CRUD by using separate API clients
• retry pod deletion, confirm the pod disappears, and log cleanup failures instead of swallowing them
• add regression tests for cleanup retries and isolated stream client usage

Testing

• uv run pytest code-interpreter/tests/integration_tests/test_kubernetes_streaming.py -q