feat: refine admin shell and model fallback

backend-fastapi/ai_platform/services/agent_service.py

@@ -241,8 +241,6 @@ class AgentService:
         if not model:
             return None
 
-        agent.model_id = model.id
-        await self._db.flush()
         logger.info(
             "Agent %s has no active model, fallback to default chat model %s",
             getattr(agent, "code", agent.id),