feat: implement pod snapshot method

[shrutiyam-glitch]

Depends on: #338 #337

This PR completes the Pod Snapshot extension for the Python SDK by implementing the snapshot() method and the underlying logic to monitor snapshot completion.

Core Logic Implementation:

• Manual Trigger Workflow: The snapshot() method creates a PodSnapshotManualTrigger resource for the target pod, using a unique hash suffix to avoid name collisions.
• Snapshot Status Watcher: Implements _wait_for_snapshot_processed, which uses the Kubernetes watch API to block until the snapshot controller marks the trigger as Complete. It specifically looks for the Triggered condition with a True status and Complete reason.
• Response Handling: Captures the snapshot_uid from the resource status and returns a structured SnapshotResponse, providing the user with the unique identifier needed for future restores.
• Lifecycle Cleanup: Updates the __exit__ method to ensure that all PodSnapshotManualTrigger resources created during the session are automatically deleted, maintaining a clean namespace.

Testing Done:

• Integration Test: Added test_podsnapshot_extension.py which verifies the full E2E flow:
  -- Starts a sandbox with a counter application.
  -- Creates two sequential snapshots (test-snapshot-10 at 10 seconds and test-snapshot-20 at 20 seconds).

• Unit tests are added

Output:

python3 test_podsnapshot_extension.py --labels app=agent-sandbox-workload --template-name python-counter-template --namespace sandbox-test
--- Starting Sandbox Client Test (Namespace: sandbox-test, Port: 8888) ---

***** Phase 1: Starting Counter *****

======= Testing Pod Snapshot Extension =======
Creating first pod snapshot 'test-snapshot-10' after 10 seconds...
Trigger Name: test-snapshot-10-890451fd
Snapshot UID: 8a72cd17-7e70-4c3c-beb2-f62ae642faed
Success: True
Error Code: 0
Error Reason: 

Creating second pod snapshot 'test-snapshot-20' after 10 seconds...
Trigger Name: test-snapshot-20-ce4fe530
Snapshot UID: a15ee9e1-5939-4110-a1e9-ce4828874734
Success: True
Error Code: 0
Error Reason: 
Recent snapshot UID: a15ee9e1-5939-4110-a1e9-ce4828874734

--- Pod Snapshot Test Passed! ---

--- Sandbox Client Test Finished ---

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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

Hi @shrutiyam-glitch. Thanks for your PR.

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[janetkuo]

/ok-to-test