[Bug] Proxy client settings sync log prints full gRPC Settings

## Problem

`ClientActivity` logs the full gRPC `Settings` protobuf when saving client settings synced from another proxy.

Evidence:
- `proxy/src/main/java/org/apache/rocketmq/proxy/grpc/v2/client/ClientActivity.java` logs `settings:{}` in the client settings sync path.

## Impact

This is in the RocketMQ Studio Track 2 / Proxy Admin diagnostics scope. Full `Settings` can include detailed publish/subscription metadata and should not be dumped into logs when a compact diagnostic summary is enough.

## Expected

The log should keep useful diagnostic fields such as client type and subscription count, while avoiding full protobuf output.

Start in proxy/src/main/java/org/apache/rocketmq/proxy/grpc/v2/client/ClientActivity.java, at the client settings sync path that logs settings. Inspect the existing diagnostic fields, retain useful values such as client type and subscription count, and verify that the full gRPC Settings protobuf is no longer emitted.

Written by the indexing model from the issue text.