refactor: migrate SelectedNetworkController to @metamask/messenger

[cryptodev-2s]

Explanation

This PR migrates SelectedNetworkController 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

---

Note

Migrates SelectedNetworkController to the new @metamask/messenger API, updates action/event typing via base-controller/next, renames metadata flag, and updates tests/config/docs accordingly.

• SelectedNetworkController:
  • Migrate to @metamask/messenger Messenger; replace messagingSystem calls and handlers with messenger.
  • Adopt @metamask/base-controller/next types (ControllerGetStateAction, ControllerStateChangeEvent).
  • Update action/event/messenger types; keep get/setNetworkClientIdForDomain behavior and subscriptions.
  • Rename state metadata anonymous -> includeInDebugSnapshot.
• Middleware/Tests:
  • Refactor tests to construct a root messenger and delegate the controller namespace; update middleware tests to use the new messenger.
• Package/Config/Docs:
  • Add dependency @metamask/messenger and reference it in tsconfig project refs.
  • Update CHANGELOG.md with BREAKING changes (new messenger; metadata rename).
  • Refresh README dependency graph to show selected_network_controller --> messenger (and related edge additions).

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[mcmire]

LGTM!