fix(ws): drop upgrades arriving after closeWebSocket instead of crashing

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.

server/websocket.js

@@ -29,6 +29,14 @@ function initWebSocket(server) {
   server.on("upgrade", (req, socket, head) => {
     const url = new URL(req.url, "http://localhost");
     if (url.pathname !== "/ws") return; // not ours — `/ws-pty/*` handles its own path.
+    // closeWebSocket() nulls `wss` but cannot unregister this listener, so an
+    // upgrade arriving mid-shutdown would throw an unhandled TypeError and take
+    // the process down instead of letting it exit gracefully. Clients retry, so
+    // dropping the socket is the correct answer here.
+    if (!wss) {
+      socket.destroy();
+      return;
+    }
     wss.handleUpgrade(req, socket, head, (ws) => {
       wss.emit("connection", ws, req);
     });
@@ -118,6 +126,12 @@ function initPtyWebSocket(server) {
       socket.destroy();
       return;
     }
+    // Same shutdown race as `/ws` above — and far easier to hit here, since
+    // TerminalView re-attaches every 1.5s while a run console is open.
+    if (!ptyWss) {
+      socket.destroy();
+      return;
+    }
     ptyWss.handleUpgrade(req, socket, head, (ws) => {
       ptyWss.emit("connection", ws, match[1]);
     });