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).