chore(lint): Fix suppressed ESLint errors in remote-feature-flag-controller package

[Mrtenz]

Explanation

This fixes all suppressed ESLint errors in the remote-feature-flag-controller package.

References

Closes #7388.

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

---

Note

Adds explicit types and a private method refactor to the client config service, tightens test typings/utilities, and removes related ESLint suppressions.

• Remote Feature Flag Controller:
  • ClientConfigApiService (client-config-api-service.ts):
    • Add explicit return types for onBreak/onDegraded (IDisposable).
    • Refactor flattenFeatureFlags to private field method #flattenFeatureFlags and update call site.
  • Controller (remote-feature-flag-controller.ts):
    • Add explicit Promise<void> return type to #updateCache.
• Tests:
  • ClientConfigApiService test: Import jest.Mock, type createMockFetch to return Mock, adjust Promise params to satisfy lint.
  • RemoteFeatureFlagController test: Annotate createController return type; type getMetaMetricsId default.
• Tooling:
  • Remove corresponding entries for this package from eslint-suppressions.json.

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[cryptodev-2s]

LGTM!