fix(search): treat HTTP 432 and plan limit errors as transient cooldown

[HouMinXi]

• Classify HTTP 432 and plan limit exhaustion strings as non-permanent QUOTA_EXHAUSTED
• Add PLAN_LIMIT_EXCEEDED: 432 constant and 1-hour quota cooldown fallback
• Trigger markAccountUnavailable in search executeProviderFetch strictly for quota/retriable statuses
• Enable search multi-key auto-rotation to healthy connections without deactivating accounts
• Add unit and regression tests including negative coverage for 400/401/403/404

Summary

• Describe the user-facing or operational change.

Related Issues

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Validation

Choose the change type and focused loop from the
Contribution Golden Path. The full unit suite,
Vitest, the 60% coverage gate, and the production build all run in CI on this PR (#8329):

• Change type: provider / routing / UI / i18n / CLI / DB / build-deploy / other
• Focused tests and category gates from the golden path
• npm run lint
• Reconciled with the current active release base; focused checks rerun afterward
• Production-code changes include a new or updated automated test in this PR
• SonarQube is temporarily opt-in while the private project has no quota; it is not a PR gate.

Tests Added Or Updated

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• If no production code changed, state that here.

Coverage Notes

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• If coverage moved down in any touched file, explain why and what follow-up task will recover it.

Reviewer Notes

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