refactor: migrate assets controllers messenger

[mikesposito]

Explanation

This PR migrates all remaining assets controllers to @metamask/messenger instead of @metamask/base-controller. This is part of a larger effort to migrate all controllers to use @metamask/messenger.

References

• Related to Implement messenger delegation #5626

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, highlighting breaking changes as necessary
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes

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Note

Migrates remaining assets controllers and tests from base-controller RestrictedMessenger to @metamask/messenger, updating metadata flags and event wiring.

• Controllers (migration to @metamask/messenger):
  • MultichainAssetsController, MultichainBalancesController, NftDetectionController, RatesController, TokenSearchDiscoveryDataController now use Messenger types and this.messenger for call/subscribe/publish (replacing RestrictedMessenger/messagingSystem).
  • Update controller-specific messenger types and allowed actions/events; add namespaced messengers in tests via delegate.
• Metadata updates:
  • Switch to @metamask/base-controller/next and StateMetadata; replace anonymous with includeInDebugSnapshot where applicable.
• Tests:
  • Refactor to root messenger + delegated namespaced messengers; update imports to /next; adjust assertions to new metadata flags; minor test helpers tweaks.
• RatesController:
  • Maintain polling events via this.messenger.publish and expose metadata via StateMetadata.
• Changelog:
  • Consolidate BREAKING note: controllers migrated to new Messenger API.

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[cryptodev-2s]

LGTM!

The base branch was changed.