Function open_and_await_hello 

async fn open_and_await_hello(
    app_token: &str,
    http: &Client,
) -> Result<WsHalf, String>

Fetches a fresh Socket Mode URL and connects, waiting for the mandatory first hello frame (spec §2.1, §4.1). Used both for the initial connect and for the cutover half of the make-before-break dance.