feat: 添加更详细的chunk日志，分析海量chunk的内容

backend/app/agent/agent_service.py

@@ -177,6 +177,14 @@ class AIAgentService:
                     message_chunk, metadata = chunk["data"]
                     node_name = metadata.get("langgraph_node", "unknown")
                     info(f"📨 处理消息chunk, node: {node_name}")
+                    # 详细记录消息内容，看看这些 chunk 到底是什么
+                    if hasattr(message_chunk, "content"):
+                        content_preview = str(message_chunk.content)[:200]
+                        info(f"📄 消息内容预览 ({len(content_preview)} chars): {repr(content_preview)}")
+                    if hasattr(message_chunk, "type"):
+                        info(f"📋 消息类型: {message_chunk.type}")
+                    if hasattr(message_chunk, "tool_calls"):
+                        info(f"🔧 包含工具调用: {message_chunk.tool_calls}")
 
                     # 检测节点变化，发送节点开始事件
                     if node_name != current_node: