closeWebSocket() nulls both WebSocketServer references but cannot unregister
the `server.on("upgrade")` listeners that initWebSocket/initPtyWebSocket
installed on the shared http.Server. An upgrade landing in that window threw
TypeError: Cannot read properties of null (reading 'handleUpgrade')
from the listener — unhandled, so it killed the process mid-SIGTERM instead of
letting it exit gracefully, and no server came back up. Observed in
runtime/server.log right after a restart, with the dashboard then showing
"Mất kết nối" and nothing else.
The previous commit's TerminalView reconnect makes this near-certain rather
than rare: every open run console re-attaches to /ws-pty every 1.5s, so a
shutdown almost always has an upgrade in flight. Both listeners now destroy
the socket when their server is gone; clients retry, which is the correct
answer during a shutdown.
Regression test uses a real http.Server and raw upgrade requests — without the
guard the process dies and the test run hangs rather than reporting a failure.
