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

Migrate MultichainAccountService and tests from @metamask/base-controller messenger to the new @metamask/messenger, updating types, helpers, configs, and dependencies.

• Multichain Account Service (BREAKING)
  • Replace RestrictedMessenger from @metamask/base-controller with new Messenger from @metamask/messenger in src/types.ts and all usages.
  • Add RootMessenger test helper and implement delegation-based getMultichainAccountServiceMessenger in src/tests/messenger.ts.
  • Update tests to use rootMessenger for registerActionHandler/publish and pass restricted messenger to providers/service.
• Dependencies/Config
  • Add dependency @metamask/messenger to packages/multichain-account-service/package.json and reference it in tsconfig*.json.
  • Update README dependency graph to include multichain_account_service --> messenger.
• Changelog
  • Document breaking change: service now uses @metamask/messenger and expects its Messenger type.

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