fix: when cot_agent call tool like searxng lost some response content (#16781)

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:
-                result = json.dumps(
+                result += json.dumps(
                     cast(ToolInvokeMessage.JsonMessage, response.message).json_object, ensure_ascii=False
                 )
             else: