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/cli/utils/test_service_factory.py

@@ -168,7 +168,8 @@ async def test_create_session_service_respects_app_name_mapping(
   assert (agent_dir / ".adk" / "session.db").exists()
 
 
-def test_create_session_service_fallbacks_to_database(
+@pytest.mark.asyncio
+async def test_create_session_service_fallbacks_to_database(
     tmp_path: Path, monkeypatch
 ):
   registry = mock.create_autospec(ServiceRegistry, instance=True, spec_set=True)
@@ -189,6 +190,7 @@ def test_create_session_service_fallbacks_to_database(
       agents_dir=str(tmp_path),
       echo=True,
   )
+  await service.close()
 
 
 def test_create_artifact_service_uses_registry(tmp_path: Path, monkeypatch):