Fix Cosmos DB health check: ensure query is actually executed

[Copilot]

Resolves #4926

What is being addressed

container.query_items("SELECT TOP 1 * FROM c") returns a lazy iterator and never executes the query, causing the health check to falsely report OK even when Cosmos DB is down or inaccessible.

How is this addressed

• health_checker.py: Use async for with break to force execution of the query, confirming connectivity:

  async for _ in container.query_items("SELECT TOP 1 * FROM c", max_item_count=1):
      break

• test_health_checker.py:
  • Fixed test_get_state_store_status_responding to patch Database.get_container_proxy (instead of ContainerProxy.query_items) and return a container mock with a real async iterator
  • Added AsyncIteratorWithError helper to simulate exceptions raised during iteration
  • Added test_get_state_store_status_cosmos_http_error and test_get_state_store_status_service_request_error to cover error paths that only trigger during actual query execution
  • Moved helper classes (AsyncIterator, AsyncIteratorWithError) to top of file
• Updated CHANGELOG.md and bumped API version to 0.25.17