chore: Fix suppressed errors in @metamask/accouncement-controller

[Gudahtt]

Explanation

Fix suppressed lint errors in @metamask/announcement-controller.

References

Closes #7341

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

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Note

Fixes a suppressed lint error by enforcing explicit return types in the announcement controller tests.

• In AnnouncementController.test.ts: add AnnouncementControllerMessenger import and set getMessenger(): AnnouncementControllerMessenger; simplify new Messenger construction with type inference
• Update eslint-suppressions.json: remove suppression entry for AnnouncementController.test.ts

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