chore(lint): Fix suppressed ESLint errors in polling-controller package

[Mrtenz]

Explanation

This fixes all suppressed ESLint errors in the polling-controller package.

References

Closes #7383.

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 return types and targeted ESLint-disable annotations to polling controller mixins and tests, removing related suppressions.

• Polling Controller:
  • BlockTrackerPollingController (packages/polling-controller/src/BlockTrackerPollingController.ts):
    • Add explicit return types to _startPolling and _stopPollingByPollingTokenSetId.
    • Document/annotate mixin factory and exported helpers with ESLint disables for inferred return types.
  • StaticIntervalPollingController (packages/polling-controller/src/StaticIntervalPollingController.ts):
    • Add explicit return types to setIntervalLength, getIntervalLength, _startPolling, and _stopPollingByPollingTokenSetId.
    • Document/annotate mixin factory and exported helpers with ESLint disables for inferred return types.
  • Tests (packages/polling-controller/src/StaticIntervalPollingController.test.ts):
    • Rename callback param err to error in deferred promise handlers.
• ESLint Suppressions:
  • Remove suppressions for updated polling-controller files in eslint-suppressions.json.

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[cryptodev-2s]

LGTM!

[cryptodev-2s]

LGTM!