WebSocket: Network error cause missing

Bug Description

Regression of #4273. The test https://github.com/nodejs/undici/blob/4f4bf4f1a6e985e7144be5b142c84e9a4823e3de/test/websocket/issue-4273.js#L13 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

Start by running test/websocket/issue-4273.js at the referenced revision and compare its passing behavior with the provided WebSocket reproduction. Trace the error handling involved in the connection refusal; done means the emitted error remains a TypeError and its cause contains the ECONNREFUSED network error.