fix: Stop FeatureStoreClientWrapper poller on close

[jsonbailey]

Note

Prevents background availability polling after shutdown and clarifies store shutdown expectations.

• Add __closed state to FeatureStoreClientWrapper, guard __update_availability when closed
• Implement close() to stop any active RepeatingTask poller and forward close() to the underlying FeatureStore if present
• Update interfaces.py with documentation/comments for optional FeatureStore.close() hook for resource cleanup

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