[Bug] Avoid exposing RemoteChannel extendAttribute in toString

### Problem

`RemoteChannel.toString()` currently includes the complete `extendAttribute` value. For gRPC clients this attribute may contain serialized channel settings or other runtime metadata. If a `RemoteChannel` is included in logs or diagnostics, the full attribute can be exposed unintentionally.

### Evidence

`proxy/src/main/java/org/apache/rocketmq/proxy/processor/channel/RemoteChannel.java` currently renders:

`extendAttribute` directly in `toString()`.

### Expected behavior

`RemoteChannel.toString()` should keep useful diagnostics such as channel id, protocol type, and remote proxy IP, but should not include the raw `extendAttribute` content. A presence flag and length are sufficient for debugging most cases.

### Scope

This is in the Proxy / Studio track scope for safer client/runtime diagnostics. The proposed fix is a small logging/stringification hardening change and does not change channel serialization, decoding, or runtime behavior.

Start in proxy/src/main/java/org/apache/rocketmq/proxy/processor/channel/RemoteChannel.java and inspect its toString() implementation. Keep the channel id, protocol type, and remote proxy IP in the diagnostics while ensuring the raw extendAttribute content is absent; a presence flag and length are sufficient.

Written by the indexing model from the issue text.