[Studio] Add consumer stack diagnostics API

[Aias00]

Summary

• add a structured Consumer stack trace response model
• introduce a Consumer diagnostics provider/service boundary for future Remoting or Proxy Admin implementations
• expose GET /api/groups/{name}/instances/{clientId}/stack
• add service and controller tests for the new read-only diagnostics contract

Motivation

This advances the Studio CLIENT-01 diagnostics surface without touching the Consumer page while #518 is open. The endpoint gives the frontend and future Proxy Admin integration a stable contract for querying a consumer instance stack trace.

Related issue: #205

Verification

• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn -B -ntp -Dtest=ConsumerDiagnosticsServiceTest,ConsumerGroupControllerTest test
• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn -B -ntp test

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