refactor: migrate MultichainAccountService to @metamask/messenger

[cryptodev-2s]

Explanation

This PR migrates MultichainAccountService to the new @metamask/messenger message bus, as opposed to the one exported from @metamask/base-controller.

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

Replaces base-controller RestrictedMessenger with the new @metamask/messenger across service, tests, and types, adding delegation-based messaging and updating configs.

• Messaging migration (BREAKING)
  • Replace @metamask/base-controller Messenger/RestrictedMessenger with @metamask/messenger.
  • Implement namespaced messenger via getRootMessenger() and delegated getMultichainAccountServiceMessenger() with explicit allowed actions/events.
  • Introduce type RootMessenger and update MultichainAccountServiceMessenger to use the new Messenger types.
• Tests
  • Update all tests to use new messenger API (rootMessenger.publish/registerActionHandler, new types) and adjust constructor wiring.
• Types
  • Update src/types.ts to use @metamask/messenger types; narrow AllowedActions/AllowedEvents to internal types and update messenger signature.
• Package/Config
  • Add dependency @metamask/messenger and reference it in tsconfig project references.
  • Changelog: note BREAKING switch to @metamask/messenger.

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