Function refresh_with_registration 

pub async fn refresh_with_registration(
    client: &Client,
    token_endpoint: &str,
    allow_http_token_endpoint: bool,
    registration: &GeminiRegistration,
    refresh: &str,
) -> Result<OAuthCredentials, String>

Refresh grant with an overridable token endpoint. Public for zero-network harness use only — production callers must go through refresh_token.