[Feature Request] Add thorough end-to-end test for async Sandbox client

[SHRUTI6991]

Currently, we miss e2e tests for AsyncSandboxClient similar to what we have for sync SandboxClient: https://github.com/kubernetes-sigs/agent-sandbox/blob/main/clients/python/agentic-sandbox-client/test_client.py.

The tests should focus on the aspect that calls will be non blocking. For example:

1. Concurrent Sandbox Creation Test - Creating multiple sandboxes at the same time. If the client is non-blocking, creating three sandboxes should take max_creation_time(s1, s2, s3).
2. Test I/O Interleaving - Client doesn't "freeze" the event loop while waiting for Kubernetes