registerNativeProvider() did not update the auth snapshot, so the provider
stayed unconfigured until an async refresh saved a snapshot that included it.
At startup that refresh can resolve before its result is saved, so initial
model selection saw the provider as unconfigured and fell back to another
provider or warned that no models are available.
Share registerProvider's provisional entry through markProvisionallyConfigured()
so native providers with a stored credential or a models.json apiKey are
configured immediately.
fixes#9962