chore: Expose missing AssetsController methods through the messenger

[GuillaumeRx]

Explanation

This exposes missing methods used in the clients through the messenger.

References

Progresses https://consensyssoftware.atlassian.net/browse/WPC-989

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

Low Risk
Low risk: this only expands the messenger-exposed API surface to include an existing setSelectedCurrency method, with no changes to business logic or data handling.

Overview
Exposes AssetsController.setSelectedCurrency via the messenger. This adds the AssetsController:setSelectedCurrency action type, includes setSelectedCurrency in the controller’s MESSENGER_EXPOSED_METHODS, exports the new action type from index.ts, and unregisters the handler on destroy().

Updates the @metamask/assets-controller changelog to document the newly exposed action.

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