Fix remote app-server shutdown race (#18936)

## Why

A Mac Bazel CI run saw `remote_notifications_arrive_over_websocket` fail
during shutdown with `remote app-server shutdown channel is closed`
(https://app.buildbuddy.io/invocation/9dac05d6-ae20-40f9-b627-fca6e91cf127).
The remote websocket worker can legitimately finish while `shutdown()`
is waiting for the shutdown acknowledgement: after the test server sends
a notification and exits, the worker may deliver the required disconnect
event, observe that the caller has dropped the event receiver, and exit
before it sends the shutdown one-shot.

That state is already terminal cleanup, not a failed shutdown, so
callers should not see a `BrokenPipe` from the acknowledgement channel.

## What Changed

- Treat a closed remote shutdown acknowledgement as an already-exited
worker while still propagating websocket close errors when the worker
returns them.
- Added a deterministic regression test for the interleaving where the
shutdown command is received and the worker exits before replying.

## Verification

- `cargo test -p codex-app-server-client`
- New test:
`remote::tests::shutdown_tolerates_worker_exit_after_command_is_queued`

codex-rs/app-server-client/src/remote.rs

@@ -612,14 +612,9 @@ impl RemoteAppServerClient {
             .send(RemoteClientCommand::Shutdown { response_tx })
             .await
             .is_ok()
-            && let Ok(command_result) = timeout(SHUTDOWN_TIMEOUT, response_rx).await
+            && let Ok(Ok(close_result)) = timeout(SHUTDOWN_TIMEOUT, response_rx).await
         {
-            command_result.map_err(|_| {
-                IoError::new(
-                    ErrorKind::BrokenPipe,
-                    "remote app-server shutdown channel is closed",
-                )
-            })??;
+            close_result?;
         }
 
         if let Err(_elapsed) = timeout(SHUTDOWN_TIMEOUT, &mut worker_handle).await {
@@ -981,4 +976,24 @@ mod tests {
             skipped: 1
         }));
     }
+
+    #[tokio::test]
+    async fn shutdown_tolerates_worker_exit_after_command_is_queued() {
+        let (command_tx, mut command_rx) = mpsc::channel(1);
+        let (_event_tx, event_rx) = mpsc::channel(1);
+        let worker_handle = tokio::spawn(async move {
+            let _ = command_rx.recv().await;
+        });
+        let client = RemoteAppServerClient {
+            command_tx,
+            event_rx,
+            pending_events: VecDeque::new(),
+            worker_handle,
+        };
+
+        client
+            .shutdown()
+            .await
+            .expect("shutdown should complete when worker exits first");
+    }
 }