Studio: add codex_router to test_desktop_auth router stub

test_health_response_reports_desktop_capability_fields builds a
SimpleNamespace as a fake routes module so it can exercise
main.health_check without standing the full app up. The stub
listed every router name except codex_router, which lands in the
main.py import block alongside the others as of this PR, so the
import failed with 'cannot import name codex_router from <unknown
module name>' on the Python 3.13 unit run.

Add the codex_router slot to the stub.

studio/backend/tests/test_desktop_auth.py

@@ -432,6 +432,7 @@ def test_health_response_reports_desktop_capability_fields(monkeypatch):
     router_stub = SimpleNamespace(
         auth_router = APIRouter(),
         chat_history_router = APIRouter(),
+        codex_router = APIRouter(),
         data_recipe_router = APIRouter(),
         datasets_router = APIRouter(),
         export_router = APIRouter(),