fix: add missing "from fastapi import APIRouter" in _agent_helpers.py

api/_agent_helpers.py

@@ -42,6 +42,7 @@ except Exception:
     async def _tg_start(*_a, **_kw): pass  # type: ignore[misc]
     async def _tg_step(*_a, **_kw):  pass  # type: ignore[misc]
 
+from fastapi import APIRouter
 router = APIRouter()
 
 @router.post('/run_loop', deprecated=True)