refactor(tool-engine): Optimize tool engine response handling (#14216)

Co-authored-by: xudong2.zhang <xudong2.zhang@zkh.com>
Co-authored-by: crazywoola <427733928@qq.com>

api/core/tools/tool_engine.py

                     + "you do not need to create it, just tell the user to check it now."
                 )
             elif response.type == ToolInvokeMessage.MessageType.JSON:
-                text = json.dumps(cast(ToolInvokeMessage.JsonMessage, response.message).json_object, ensure_ascii=False)
-                result += f"tool response: {text}."
+                result = json.dumps(
+                    cast(ToolInvokeMessage.JsonMessage, response.message).json_object, ensure_ascii=False
+                )
             else:
-                result += f"tool response: {response.message!r}."
+                result += str(response.message)
 
         return result