[Studio] fix: query producer connections with producer group

[yx9o]

What is the purpose of the change

Fix producer connection queries so Studio passes the required producer group and topic to the RocketMQ admin API, avoiding silent empty results on RocketMQ 5.3.3.

Brief changelog

• Require both topic and producer group for producer connection queries.
• Pass producerGroup and topic to examineProducerConnectionInfo.
• Discover producer clients from Broker producer tables for the general client list.
• Return HTTP 400 for missing parameters and HTTP 502 for RocketMQ query failures.
• Add backend and frontend regression tests for the updated behavior.

Verifying this change

• cd server && mvn -Dtest=ProducerConnectionServiceTest,ProducerControllerTest,RocketMQClientProviderTest,ClientProviderStubTest test
• cd server && mvn test
• cd server && mvn -DskipTests package
• cd web && npm test -- src/api/producer.test.ts src/pages/studio/__tests__/Producer.test.tsx --maxWorkers=1 --no-file-parallelism
• cd web && npm test -- --maxWorkers=1 --no-file-parallelism
• cd web && npm run build
• git diff --check