Function check_armed_pool 

async fn check_armed_pool(
    client: &Client,
    gcp: &GcpApiClient,
    config: &Config,
    pool: &str,
    last_triggered: &mut BTreeMap<(String, String), Instant>,
) -> Result<PoolCheckOutcome, Error>

Checks the upgrade progress of a single armed node pool, triggering rollouts for affected instances once the blue pool is fully cordoned.