Fix deadlock in handle_send_to_dead_session test

The test called handle.join() without first calling handle.close(), causing the spawned session task to park forever waiting for cmd_rx.recv() to return None. Since join() borrows &self, cmd_tx stays alive, and the task never exits. Add close() call before join() to send a Close command and unblock the task.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

crates/xserial-client/src/session.rs

@@ -576,6 +576,7 @@ mod tests {
         let handle = Session::spawn(2, tcp_config());
         tokio::time::sleep(Duration::from_millis(200)).await;
         let _ = handle.send(b"data".to_vec()).await;
+        let _ = handle.close().await;
         handle.join().await;
     }