feat: 添加 finalize 节点实现

backend/app/main_graph/nodes/finalize.py

@@ -0,0 +1,54 @@
+"""
+Finalize 节点 - 轻量后处理
+"""
+
+from typing import Dict, Any, Optional
+from langchain_core.runnables.config import RunnableConfig
+from langchain_core.messages import AIMessage
+
+from backend.app.main_graph.state import AgentState
+from backend.app.logger import info
+
+
+async def finalize_node(state: AgentState, config: Optional[RunnableConfig] = None) -> Dict[str, Any]:
+    """
+    Finalize 节点：轻量后处理
+
+    职责：
+    1. 从 messages 中提取最后一条 AIMessage.content 作为最终回复
+    2. 汇总元数据：步数、使用的工具、停止原因
+    3. 如果 final_reply 为空且有 stop_reason，生成说明文本
+    """
+    info("[Finalize] 执行后处理")
+
+    # 获取最终回复
+    final_reply = ""
+    for msg in reversed(state.messages):
+        if isinstance(msg, AIMessage) and msg.content:
+            final_reply = msg.content
+            break
+
+    # 获取停止原因
+    stop_reason = getattr(state, "stop_reason", "")
+    if not final_reply and stop_reason:
+        # 如果没有回复但有停止原因，生成说明
+        reason_messages = {
+            "loop_detected": "[系统] 检测到工具调用循环，已终止。",
+            "max_steps": "[系统] 已达到最大步数限制。",
+        }
+        final_reply = reason_messages.get(stop_reason, f"[系统] 已终止，原因: {stop_reason}")
+
+    # 汇总元数据
+    metadata = {
+        "steps_taken": getattr(state, "current_step", 0),
+        "tools_used": getattr(state, "tools_used", []),
+        "stop_reason": stop_reason,
+        "llm_calls": getattr(state, "llm_calls", 0),
+    }
+
+    info(f"[Finalize] 完成 - steps: {metadata['steps_taken']}, tools: {len(metadata['tools_used'])}")
+
+    return {
+        "final_reply": final_reply,
+        "metadata": metadata,
+    }