Function poll_device_token_once 

pub async fn poll_device_token_once(
    http: &Client,
    token_endpoint: &str,
    device_code: &str,
    client_id: &str,
    resource: Option<&str>,
) -> Result<DevicePollOutcome, OAuthError>

Poll the token endpoint once with the device_code grant (RFC 8628 §3.4/§3.5). authorization_pending/slow_down map to non-terminal outcomes; other error codes (access_denied, expired_token, …) surface as OAuthError::Oauth.