[Bug] Avoid logging full system message data on proxy sync failures

### Problem `AbstractSystemMessageSyncer.sendSystemMessage` logs the full `data` object when sending a system broadcast message fails. The serialized system message payload can contain client/runtime metadata, so failure logs should avoid dumping the whole object. This is similar to the recent telemetry-command log hardening work: the proxy should preserve useful diagnostics without exposing complete payload objects in error logs. ### Evidence `proxy/src/main/java/org/apache/rocketmq/proxy/service/sysmessage/AbstractSystemMessageSyncer.java` currently logs: - `send system message failed. data: {}, topic: {}` when async send throws - `send system message failed. data: {}, topic: {}, sendResult:{}` when the send result is not OK - the same full `data` object in the outer catch block ### Expected behavior The error log should record a compact summary such as data type and topic, and avoid rendering the complete system message object. This keeps operational diagnostics while reducing accidental metadata/payload exposure in proxy logs. ### Scope This is in the Proxy / Studio track scope for safer Proxy Admin/runtime diagnostics. The proposed fix is a small logging hardening change and does not change system message serialization or delivery behavior.