Function run_session 

async fn run_session(
    token: &str,
    channel_id: &str,
    target_agent: &Option<String>,
    connect_url: &str,
    http: &Client,
    outbound_rx: &mut UnboundedReceiver<OutboundMsg>,
    health: &Arc<Mutex<BridgeHealth>>,
    resume_session: Option<Session>,
) -> Result<Option<Session>, String>

Run a single Gateway session. Returns: Ok(Some(session)) — clean disconnect, can attempt RESUME Ok(None) — clean disconnect, session is invalid (re-IDENTIFY) Err(e) — unexpected error, will re-IDENTIFY after backoff