feat: 新增 react_reason 循环思考过程的流式显示

- 修改 react_nodes.py，在推理时保存推理过程到状态
- 修改 agent_service.py，检测并发送推理过程事件到前端
- 修改 chat_area.py，接收并显示推理过程
- 修改 useChat.ts，添加对推理过程事件的支持

backend/app/agent/agent_service.py

@@ -258,6 +258,34 @@ class AIAgentService:
                     info(f"🔄 处理updates chunk")
                     updates_data = chunk["data"]
                     serialized_data = self._serialize_value(updates_data)
+                    
+                    # 检查是否有最新的推理内容（来自 react_reason_node）
+                    if isinstance(serialized_data, dict):
+                        # 遍历所有节点的数据
+                        for node_name, node_data in serialized_data.items():
+                            if isinstance(node_data, dict) and "debug_info" in node_data:
+                                debug_info = node_data["debug_info"]
+                                latest_reasoning = debug_info.get("latest_reasoning")
+                                if latest_reasoning and not latest_reasoning.get("sent"):
+                                    # 发送推理过程到前端
+                                    step = latest_reasoning.get("step", 1)
+                                    action = latest_reasoning.get("action", "unknown")
+                                    confidence = latest_reasoning.get("confidence", 0)
+                                    reasoning = latest_reasoning.get("reasoning", "")
+                                    
+                                    info(f"[Agent Service] 发送推理过程 #{step}: {action}")
+                                    
+                                    # 发送推理事件
+                                    yield {
+                                        "type": "react_reasoning",
+                                        "step": step,
+                                        "action": action,
+                                        "confidence": confidence,
+                                        "reasoning": reasoning
+                                    }
+                                    
+                                    # 标记为已发送（避免重复发送）
+                                    latest_reasoning["sent"] = True
 
                     # 检查是否有人工审核请求
                     if "review_pending" in serialized_data and serialized_data["review_pending"]: