fix: Messenger type errors in ComposableController

[MajorLift]

Explanation

• Fix incompatibility of ChildControllerStateChangeEvents type with BaseController (when used in the Events type argument of ComposableControllerMessenger) by removing unnecessary nested logic from definition.
• Update generic parameter names ControllerName and ControllerState to ChildControllerName, ChildControllerState for reduced ambiguity.

References

• Branches from refactor: migrate ComposableController to @metamask/messenger #6710

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

Simplifies ChildControllerStateChangeEvents and types the messenger subscribe call to fix ComposableController Messenger type errors; updates changelog.

• Composable Controller:
  • Types: Simplify ChildControllerStateChangeEvents by removing nested conditional logic and renaming generics to ChildControllerName/ChildControllerState.
  • Messenger: Type the subscribe call for child stateChange events, updating composed state with the received child state (with an inline ts-expect-error to bypass an unnecessary overload constraint).
• Docs:
  • Update packages/composable-controller/CHANGELOG.md with a Fixed entry describing the type compatibility change.

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