fix(mock): isolate consumer groups by instance

[btlqql]

Summary

• retain instanceId on newly created mock consumer groups
• scope mock list, detail, refresh and delete operations to the requested instance
• allow equal group names in different instances while rejecting duplicates within one instance

Tests

• npm test -- src/services/consumerService.test.ts (16 passed)
• npm exec eslint -- src/services/consumerService.ts src/services/consumerService.test.ts
• git diff --check

Closes #2878

[RockteMQ-AI]

Summary

Fixes mock data to isolate consumer groups by instance name, preventing cross-contamination between test scenarios. Small, focused change with proper test coverage.

LGTM

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