[composable-controller] Fix incorrect `#updateChildController` behavior and improve typing

[MajorLift]

There are several issues with the current ComposableController implementation that should be addressed before further updates are made to the controller (e.g. #3627).

• Remove #controllers class field, which is not being updated by #updateChildController or anywhere else.
  • Removing this makes it clear that the list of child controllers to be composed is determined at class instantiation and cannot be altered later.
  • This behavior is consistent with #updateChildController being a private method.
• Type BaseController, ComposableController state with Record<string, Json>
• Add a isBaseController type guard.
  • Remove the deprecated subscribed property from BaseController.