feat: 适配 finalize 节点返回的 final_reply 和 metadata

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/agent/agent_service.py

@@ -105,19 +105,24 @@ class AIAgentService:
         config, input_state = self._build_invocation(message, thread_id, resolved_model, user_id)
 
         result = await self.graph.ainvoke(input_state, config=config)
-        
-        reply = ""
-        if result.get("messages"):
+
+        # 优先使用 final_reply（finalize 节点返回）
+        reply = result.get("final_reply", "")
+        if not reply and result.get("messages"):
             reply = result["messages"][-1].content
-        
+
         token_usage = result.get("last_token_usage", {})
         elapsed_time = result.get("last_elapsed_time", 0.0)
 
+        # 获取元数据
+        metadata = result.get("metadata", {})
+
         return {
             "reply": reply,
             "token_usage": token_usage,
             "elapsed_time": elapsed_time,
-            "model_used": resolved_model
+            "model_used": resolved_model,
+            "metadata": metadata
         }
 
     async def process_message_stream(