feat: Improve asyncio loop handling and test cleanup

This CL enhances asyncio event loop management and test isolation.

-   **BigQuery Analytics Plugin:** Ensure the asyncio event loop is consistently closed within the BigQuery analytics plugin. This prevents potential resource leaks. Add checks to handle potential deadlocks in Python 3.13+ when creating loops during interpreter shutdown.
-   **Test Thread Pool Cleanup:** Introduce a pytest fixture (`cleanup_thread_pools`) to automatically shut down and clear all tool-related thread pools after each test run in `test_functions_thread_pool.py`. This improves test isolation and prevents order-dependent test failures.
-   **Streaming Test Loop Restoration:** Refactor event loop handling in `test_streaming.py`. A new `_run_with_loop` method is introduced in the custom test runners to create a temporary event loop for each test execution, run the coroutine, and crucially, restore the original event loop afterwards. This prevents tests from interfering with each other's loop state.
-   **Resource Closure:** Ensure services are closed properly in tests by adding `await service.close()` in `test_service_factory.py` and using `async with session_service` in `test_session_service.py`.

PiperOrigin-RevId: 863305565

tests/unittests/flows/llm_flows/test_functions_thread_pool.py

@@ -32,6 +32,17 @@ import pytest
 from ... import testing_utils
 
 
+@pytest.fixture(autouse=True)
+def cleanup_thread_pools():
+  yield
+  from google.adk.flows.llm_flows import functions
+
+  # Shutdown all pools
+  for pool in functions._TOOL_THREAD_POOLS.values():
+    pool.shutdown(wait=False)
+  functions._TOOL_THREAD_POOLS.clear()
+
+
 class TestIsSyncTool:
   """Tests for the _is_sync_tool helper function."""