chore: add changelog

changes/ce/fix-12814.en.md

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+Handle several errors in `/clients/{clientid}/mqueue_messages` and `/clients/{clientid}/inflight_messages` APIs:
+
+- Internal timeout, which means that EMQX failed to get the list of Inflight/Mqueue messages within the default timeout of 5 s. This error may occur when the system is under a heavy load. The API will return 500 `{"code":"INTERNAL_ERROR","message":"timeout"}` response and log additional details.
+- Client shutdown. The error may occur if the client connection is shutdown during the API call. The API will return 404 `{"code": "CLIENT_SHUTDOWN", "message": "Client connection has been shutdown"}` response in this case.