Fix remote app-server shutdown race

[bolinfest]

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