[Studio] Return dashboard mock copies

[Aias00]

Summary

• return copied dashboard stats from the mock service
• copy cluster overview records and throughput arrays before returning them
• add regression coverage for overview mock response mutation isolation

Tests

• npm --prefix web test -- --run src/services/dashboardService.test.ts
• (cd web && npm exec eslint -- src/services/dashboardService.ts src/services/dashboardService.test.ts)
• npm --prefix web run build

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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Superseded by #631.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.