fix: Built-in agents (names starting with "__") now use in-memory session storage instead of creating .adk folders in the agents directory

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 845852695

tests/unittests/cli/utils/test_local_storage.py

@@ -72,6 +72,18 @@ async def test_per_agent_session_service_respects_app_name_alias(
   assert (tmp_path / folder_name / ".adk" / "session.db").exists()
 
 
+@pytest.mark.asyncio
+async def test_per_agent_session_service_routes_built_in_agents_to_root_dot_adk(
+    tmp_path: Path,
+) -> None:
+  service = PerAgentDatabaseSessionService(agents_root=tmp_path)
+
+  await service.create_session(app_name="__helper", user_id="user")
+
+  assert not (tmp_path / "__helper").exists()
+  assert (tmp_path / ".adk" / "session.db").exists()
+
+
 def test_create_local_database_session_service_returns_sqlite(
     tmp_path: Path,
 ) -> None: