[Studio] Return topic mock copies

[Aias00]

Summary

• return copied rows from topic mock list/create/update flows
• return copied route and consumer rows from topic detail mock helpers
• add regression coverage so caller mutations do not pollute later mock reads

Tests

• npm test -- --run src/services/topicService.test.ts
• npm run lint -- --max-warnings=999 (passes with existing Fast Refresh warnings)
• 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Superseded by #631.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.