[Studio] Add DLQ provider stub data

[Aias00]

Summary

• return sample DLQ groups from the backend stub provider instead of an empty list
• support clusterId filtering in the stub data set
• add provider tests for sample data, filtering, unknown clusters, and defensive copies

Validation

• JAVA_HOME=$(/usr/libexec/java_home -v 21) mvn -Dtest=DLQProviderStubTest,DLQServiceTest,DLQControllerTest test
• JAVA_HOME=$(/usr/libexec/java_home -v 21) mvn test
• git diff --check

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[Aias00]

Superseded by #640. Consolidated with related DLQ data and UI action changes to reduce review overhead while preserving the same scoped fixes and tests.