Both WikibaseClient::newTermSearchInteractor and WikibaseRepo::newTermSearchInteractor should return a DispatchingTermSearchInteractor. They should get that instance from DispatchingServiceFactory.
Note that newTermSearchInteractor is currently guaranteed to always return a fresh instance, which does not fit with how ServiceContainer handles service instances. One solution would be to make createService() protected in ServiceContainer, and call that in DispatchingTermSearchInteractor::newTermSearchInteractor. But perhaps it's not actually required to always have a fresh interactor instance. Maybe we can change the method to lazy getter semantics, and rename it to getTermSearchInteractor. A third option is to introduce a factory.