Function run_session 

async fn run_session(
    app_token: &str,
    channel_id: &str,
    target_agent: &Option<String>,
    http: &Client,
    health: &Arc<Mutex<BridgeHealth>>,
    primary: &mut WsHalf,
) -> Result<(), String>

Runs one connection’s session loop, holding at most two live socket halves (primary + optionally old, see module doc comment).

Returns Ok(()) on a clean close (primary closed with no error) or Err(e) on anything reconnect-worthy (unexpected close, dead-connection timeout, ws send/recv error). Either way the caller (run_socket_loop) starts a fresh session with a freshly-fetched URL.