[Studio] Load producer topics after mount

[Aias00]

Summary

• move Producer topic initialization out of render and into a mount effect
• guard async topic loading so it does not update state after unmount
• add Producer page coverage for loading API-backed topic options

Scope

• RocketMQ Studio contest track 1 / BASE-01 client and producer connection management UX

Tests

• npm test -- src/pages/studio/tests/Producer.test.tsx
• npm run lint (passes with existing fast-refresh warnings outside this change)
• npm run build

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[Aias00]

Superseded by #638. Consolidated with related page mount-loading changes to reduce review overhead while preserving the same scoped fixes and tests.