refactor: migrate AccountTreeController to @metamask/messenger

[mikesposito]

Explanation

This PR migrates the AccountTreeController class to the new @metamask/messenger comm system, 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

---

Note

Migrates AccountTreeController and tests to the new @metamask/messenger API, updates state metadata and base-controller/next usage, and adds the messenger dependency.

• AccountTreeController:
  • Switch to @metamask/messenger (replace messagingSystem/restricted messenger with messenger; update subscribe/publish/call and action handler registration).
  • Use @metamask/base-controller/next; change state metadata key from anonymous to includeInDebugSnapshot.
  • Pass messenger through backup/sync context; construct rules (EntropyRule, SnapRule, KeyringRule) with messenger.
• Tests:
  • Add tests/mockMessenger to provide root and controller messengers; refactor all tests to use it; remove inline Messenger scaffolding and update spies/handlers accordingly.
• Package/Config:
  • Add dependency @metamask/messenger and reference it in tsconfig files; update README dependency graph to include account-tree-controller --> messenger.
• Changelog:
  • Document BREAKING change: controller now accepts Messenger from @metamask/messenger instead of RestrictedMessenger from @metamask/base-controller.

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[mcmire]

LGTM!

[mcmire]

LGTM!

[mcmire]

Nit: Do we want to export a RootMessenger type from mockMessenger.ts?

[cryptodev-2s]

LGTM!

[cursor]

Bug: Mock Handler Not Registered

The mockGetSelectedMultichainAccountActionHandler is defined but not registered as the handler for AccountsController:getSelectedMultichainAccount. Tests attempt to configure its return value, but the action is actually handled by mocks.AccountsController.getSelectedMultichainAccount. This means the test mocks have no effect, leading to unexpected test behavior.