[Studio] fix: validate cluster members before operations

[yx9o]

What is the purpose of the change

Cluster operations only verified that the cluster existed. Operations targeting a missing Broker, NameServer, or Proxy could still report success.

Brief changelog

• Validate the Broker before restart.
• Validate the NameServer before update, restart, upgrade, and deletion.
• Validate the Proxy before restart.
• Return a 404 business error when the target does not exist.

Verifying this change

• mvn -q -Dtest=ClusterServiceTest test
• mvn -q test
• mvn -q -DskipTests package
• mvn -q checkstyle:check