[Studio] Cover client controller contract

[Aias00]

Summary

• add WebMVC coverage for the unified client connection list endpoint
• verify Remoting and gRPC protocol fields are serialized in client rows
• verify cluster and client type filters are passed to the service layer

Scope

• RocketMQ Studio contest scope: Track 1 / CLIENT-01 dual-protocol client connection display
• Test-only change; no runtime behavior changes

Tests

• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn -q -Dtest=ClientControllerTest,ProducerControllerTest,ProducerConnectionServiceTest test
• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn -q test
• git diff --check

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[Aias00]

Superseded by #641. Folded into the consolidated runtime/client view PR to keep client runtime UI behavior and controller contract coverage in one review unit.