feat: refine admin shell and model fallback

backend-fastapi/ai_platform/nodes/builtin/intent_node.py

@@ -435,5 +435,5 @@ class IntentNode(BaseNode):
                     'default': 'intent_result',
                 },
             },
-            'required': ['model_id', 'intents'],
+            'required': ['intents'],
         }

backend-fastapi/ai_platform/nodes/builtin/llm_node.py

@@ -1065,5 +1065,5 @@ class LLMNode(BaseNode):
                     'default': 'llm_response',
                 },
             },
-            'required': ['model_id'],
+            'required': [],
         }

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),