WebSocket: Network error cause missing

[chrmarti]

Bug Description

Regression of #4273. The test

undici/test/websocket/issue-4273.js

Line 13 in 4f4bf4f

t.assert.ok(ev.error instanceof TypeError)

passes, but there is no cause with ECONNREFUSED on the error.

Reproducible By

import { WebSocket } from 'undici';

// No actual server here
const ws = new WebSocket('ws://localhost:1234');

ws.addEventListener('error', (event) => {
  console.log('TypeError', event.error instanceof TypeError);
  console.log('error.cause', event.error?.cause);
});

Expected Behavior

Error should have a cause with the network error.

Logs & Screenshots

N/A

Environment

macOS Tahoe 26.4. Node.js 22.22.1. Undici 8.0.2.

Additional context

N/A