[Studio] Return consumer mock copies

[Aias00]

Summary

• return copied consumer group, instance, progress, and subscription mock rows
• copy created consumer groups before returning them to callers
• add regression coverage for caller-side mutation isolation

Tests

• npm ci --ignore-scripts --no-audit --no-fund
• npm test -- --run src/services/consumerService.test.ts
• npm run lint -- --max-warnings=999
• git diff --check

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[Aias00]

Superseded by #631. Consolidated with related mock service hardening changes to reduce review overhead while preserving the same scoped fixes and tests.