feat: add async pod snapshot support to Python SDK

[AndyJCai]

Summary

Ports GKE Pod Snapshot capabilities from the sync client (PodSnapshotSandboxClient) to the async SDK:

• AsyncSnapshotEngine — async create, list, delete, delete_all, delete_manual_triggers
• AsyncSandboxWithSnapshotSupport — async suspend(), resume(), is_suspended(), is_restored_from_snapshot(), terminate()
• AsyncPodSnapshotSandboxClient — subclass of AsyncSandboxClient with lazy CRD verification via asyncio.Lock
• Async utilities — watch/wait helpers using kubernetes_asyncio with proper await w.close() cleanup

Also fixes:

• Pod name cache staleness on resume (sync + async) — new _resolve_pod_name() bypasses cache during polling
• False-positive termination when pod UID is None
• Resource leak if __aenter__ CRD check fails

[k8s-ci-robot]

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This pull-request has been approved by: AndyJCai
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[k8s-ci-robot]

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