langchain-community: fix unicode escaping issue with SlackToolkit (#16616)

- **Description:** fix unicode escaping issue with SlackToolkit
  - **Issue:**  #16610

libs/community/langchain_community/tools/slack/get_message.py

@@ -39,6 +39,6 @@ class SlackGetMessage(SlackBaseTool):
                 for message in messages
                 if "user" in message and "text" in message and "ts" in message
             ]
-            return json.dumps(filtered_messages)
+            return json.dumps(filtered_messages, ensure_ascii=False)
         except Exception as e:
             return "Error creating conversation: {}".format(e)