Function spawn_region_discovery 

fn spawn_region_discovery(
    profile_id: ProfileId,
    buckets: Vec<BucketSummary>,
    pool: Arc<ClientPool>,
    cache: CacheHandle,
    log: Arc<RwLock<NotificationLog>>,
    channel: AppHandle,
    default_region: String,
)

Spawn a background task that calls GetBucketLocation for every bucket in the list, updating the per-bucket region cache on success and emitting buckets:updated per discovery.