Shared base for placeholder providers that haven't been implemented yet.
Future ExAtlas.Providers.Fly, ExAtlas.Providers.LambdaLabs, ExAtlas.Providers.Vast
modules use this to reserve the provider atom in ExAtlas.Config, expose an
accurate capabilities/0 list, and fail every other callback with a clear
{:error, :not_implemented} so callers get a helpful message rather than a
FunctionClauseError.
When a real implementation lands, the provider module stops use-ing Stub
and implements the callbacks directly — no downstream call-site changes.