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_to_be_completed, 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

Follow-up PR:
Restore example, is_restored() check added in the following PR (#249)
List_snapshots, delete_snapshots and restoring from dedicated snapshot will be added in the following PRs.

Output:

• Integration Test (clients/python/agentic-sandbox-client/test_podsnapshot_extension.py):

$ python3 test_podsnapshot_extension.py --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-10-20260306-190022-b6016fa2
Snapshot UID: 69debd3e-3518-4185-8d09-720f933b2100
Success: True
Error Code: 0
Error Reason: 

Creating second pod snapshot 'test-snapshot-20' after 10 seconds...
Trigger Name: test-snapshot-20-20260306-190035-874a66a1
Snapshot UID: a10f67a6-bd84-4684-b8bd-506cde0f7757
Success: True
Error Code: 0
Error Reason: 
Recent snapshot UID: a10f67a6-bd84-4684-b8bd-506cde0f7757
--- Pod Snapshot Test Passed! ---

--- Sandbox Client Test Finished ---

• Unit test (clients/python/agentic-sandbox-client/k8s_agent_sandbox/gke_extensions/test_podsnapshot_client.py):

$ python3 k8s_agent_sandbox/gke_extensions/test_podsnapshot_client
.py
.......ERROR:k8s_agent_sandbox.gke_extensions.podsnapshot_client:Failed to create PodSnapshotManualTrigger 'test-trigger-fe4e166a': (Create failed)
Reason: None
Traceback (most recent call last):
  File "/usr/local/google/home/shrutiyam/Documents/python-test/split1/agent-sandbox/clients/python/agentic-sandbox-client/k8s_agent_sandbox/gke_extensions/podsnapshot_client.py", line 214, in snapshot
    created_obj = self.custom_objects_api.create_namespaced_custom_object(
        group=PODSNAPSHOT_API_GROUP,
    ...<3 lines>...
        body=manifest,
    )
  File "/usr/lib/python3.13/unittest/mock.py", line 1169, in __call__
    return self._mock_call(*args, **kwargs)
           ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/unittest/mock.py", line 1173, in _mock_call
    return self._execute_mock_call(*args, **kwargs)
           ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/unittest/mock.py", line 1228, in _execute_mock_call
    raise effect
kubernetes.client.exceptions.ApiException: (Create failed)
Reason: None

...ERROR:k8s_agent_sandbox.gke_extensions.podsnapshot_client:Snapshot creation timed out for trigger 'test-trigger-7faa61c6': Snapshot manual trigger 'test-trigger-7faa61c6' was not processed within 1 seconds.
Traceback (most recent call last):
  File "/usr/local/google/home/shrutiyam/Documents/python-test/split1/agent-sandbox/clients/python/agentic-sandbox-client/k8s_agent_sandbox/gke_extensions/podsnapshot_client.py", line 224, in snapshot
    snapshot_result = self._wait_for_snapshot_processed(
        trigger_name, resource_version
    )
  File "/usr/local/google/home/shrutiyam/Documents/python-test/split1/agent-sandbox/clients/python/agentic-sandbox-client/k8s_agent_sandbox/gke_extensions/podsnapshot_client.py", line 176, in _wait_for_snapshot_processed
    raise TimeoutError(
        f"Snapshot manual trigger '{trigger_name}' was not processed within {self.podsnapshot_timeout} seconds."
    )
TimeoutError: Snapshot manual trigger 'test-trigger-7faa61c6' was not processed within 1 seconds.
.
----------------------------------------------------------------------
Ran 11 tests in 1.452s

OK

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[APPROVALNOTIFIER] This PR is NOT APPROVED

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[janetkuo]

/ok-to-test

[SHRUTI6991]

/lgtm

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