Test CharacterListViewModel and CharacterDetailViewModel entirely through their
MVI surface with a FakeCharacterRepository (a fake, not a mock) and a directly
constructed SavedStateHandle, on StandardTestDispatcher. Coverage: happy path,
error -> UiText + snackbar Event, pagination end-reached, the in-flight and
duplicate next-page guards, process-death restore, and both branches of OnRetry.
Also a domain test for GetCharactersPageUseCase (delegation + error
propagation).
Add a domain UseCase (operator invoke) in :feature:characters:domain delegating
to CharacterRepository, and have CharacterListViewModel depend on it instead of
the repository directly. The UseCase is a deliberate thin pass-through that
documents the 'when to add a UseCase' convention (real logic / multi-source
composition vs. a single forwarded call).
