Function poll 

pub async fn poll<F, Fut>(
    client: &Client,
    base_url: &str,
    client_id: &str,
    device_code: &str,
    initial_interval: u64,
    expires_in: u64,
    sleep: F,
) -> Result<PollSuccess, ClientError>
where
    F: FnMut(Duration) -> Fut,
    Fut: Future<Output = ()>,

Poll {base_url}/oauth2/v2.0/token per RFC 8628 §3.5. Returns when the user has approved consent, or with an error on denial / expiry.

The sleep parameter is a closure so tests can inject a fast no-op sleep instead of real tokio::time::sleep.