OpenAI engine: requests over ~300 s fail with openai >= 5, whatever the timeout #59

engines.openai() cannot wait more than about 5 minutes for a clip when the installed openai SDK is version 5 or newer, whatever timeout the SDK is given.

From v5 the SDK uses Node's built-in fetch (undici), whose headersTimeout is 300 s by default. The SDK's own timeout (10 minutes by default) does not raise it, so a request that has not received response headers after 300 s fails with APIConnectionTimeoutError. With v4 (node-fetch) there is no such limit.

OpenAI's own API answers well within that, so this mostly affects self-hosted OpenAI-compatible servers (see the baseUrl option in #58): on CPU a long sentence can take several minutes. The peer range is openai: ">=4", so a fresh install gets v6 and hits it.

Reproduce

A stub server that answers after 320 s:

// stub.mjs
import http from 'node:http';
http.createServer((req, res) => {
  req.resume();
  setTimeout(() => { res.writeHead(200, { 'content-type': 'audio/pcm' }); res.end(Buffer.alloc(480)); }, 320_000);
}).listen(9902);

// call.mjs
import { OpenAI } from 'openai';
const client = new OpenAI({ apiKey: 'unused', baseURL: 'http://127.0.0.1:9902/v1', timeout: 1_800_000, maxRetries: 0 });
await client.audio.speech.create({ model: 'm', voice: 'v', input: 'x', response_format: 'pcm' });

Measured on Node 24, Windows:

Possible fixes (maintainer's call)

1. Let callers pass a fetch (or fetchOptions) through engines.openai(), e.g. one using an undici Agent with a larger headersTimeout. No new dependency in Argo.
2. Have the engine create an undici Agent from timeoutMs itself. Works out of the box, adds undici as a dependency.
3. Keep the documented limit (the README mentions it as of feat(tts): OpenAI engine talks to any OpenAI-compatible server #58).

Acceptance criteria

• A request through engines.openai() with openai >= 5 can wait longer than 300 s for a response when configured to, or the README states the limit and how to work around it.
• A test or reproduction shows the chosen behaviour.