[Studio] Surface consumer group Admin failures

[Aias00]

Closes #965

Summary

• preserve the successful offline fallback only for MQClientException with CONSUMER_NOT_ONLINE
• convert timeout, broker, ACL, and other Admin call failures into explicit HTTP 502 business responses
• add regression coverage for offline, timeout, and broker/ACL failure paths

Related

This is intentionally separate from #786 / #787, which cover unconfigured default providers rather than the real RocketMQAdminClientImpl Admin path.

Verification

JAVA_HOME=$(/usr/libexec/java_home -v 21) PATH="$JAVA_HOME/bin:$PATH" mvn -Dtest=RocketMQAdminClientImplTest test

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[Aias00]

Superseded by #1025. The replacement is rebased on rocketmq-studio and contains only the #965 implementation and focused regression tests; closing this PR to avoid retaining unrelated historical changes.

[Aias00]

Closed in favor of the clean replacement #1025.