[Bug] GrpcClientChannel conversion failure logs raw gRPC Settings data

## Problem

`GrpcClientChannel` logs raw gRPC Settings data when converting between Settings and channel extend attributes fails.

Evidence:
- `proxy/src/main/java/org/apache/rocketmq/proxy/grpc/v2/channel/GrpcClientChannel.java` logs the full `settings` object in `toChannelExtendAttribute`.
- The same class logs the raw `attr` string in `parseChannelExtendAttribute`.

## Impact

This is in the RocketMQ Studio Track 2 / Proxy Admin diagnostics scope. Settings and serialized channel attributes can include detailed publish/subscription metadata, so conversion failure logs should avoid dumping them.

## Expected

Failure logs should keep enough context for diagnosis, such as client type, publish/subscription counts, and attribute length, without printing raw Settings or serialized attributes.

Read proxy/src/main/java/org/apache/rocketmq/proxy/grpc/v2/channel/GrpcClientChannel.java, focusing on toChannelExtendAttribute and parseChannelExtendAttribute. Compare the failure logs with the available client type, publish/subscription counts, and attribute length. Done means conversion failures retain diagnostic context without logging the raw settings object or serialized attr string.

Written by the indexing model from the issue text.