Fix Node 26+ compatibility (UND_ERR_INVALID_ARG)

[IvanPleshkov]

Fix: Node 26+ compatibility — UND_ERR_INVALID_ARG: invalid onError method

Fixes #134

On Node 26+ every request throws TypeError: fetch failed with cause UND_ERR_INVALID_ARG: invalid onError method.

This is the "two undici in one process" problem. The REST client pinned undici@^6 and built its own undici. Agent, then passed it as init. dispatcher to Node's global fetch. On Node ≤22, the bundled undici accepted the v6 Agent; Node 26 bundles undici 8.x, whose dispatcher hook contract is stricter and rejects the v6 Agent during dispatch init.

Fix

The request no longer flows through the global fetch with a foreign dispatcher. A terminal middleware now performs the request itself with a resolved fetch, picked in this order:

A - a caller-supplied fetch (new optional fetch option on QdrantClient);
B (default on Node) - undici's own fetch paired with an undici Agent from the same package, so the dispatcher contract always matches regardless of the undici version Node ships;
C - otherwise the platform's global fetch.

Notes:

The undici-backed transport lives in its own module (src/node-fetch.ts) and is referenced only behind a process guard.
The terminal middleware re-implements the dependency's response parsing, including the bigint reviver, so large-integer handling is preserved.
Public API is unchanged except for the new optional, backward-compatible fetch option.

How to reproduce / verify

Reproduced and verified with real client builds on node:26-alpine (Node v26.3.1) against a local Qdrant-shaped HTTP server.

repro.mjs file:

import http from 'node:http';
import {QdrantClient} from './dist/esm/index.js';

const server = http.createServer((req, res) => {
    res.setHeader('content-type', 'application/json');
    res.end(req.url.endsWith('/exists')
        ? JSON.stringify({result: {exists: true}, status: 'ok', time: 0})
        : JSON.stringify({result: {collections: []}, status: 'ok', time: 0}));
});
await new Promise((r) => server.listen(0, r));
const {port} = server.address();

const client = new QdrantClient({url: `http://127.0.0.1:${port}`, checkCompatibility: false});
try {
    console.log(await client.collectionExists('my-collection'));
    console.log(await client.getCollections());
    console.log('WORKS ✓');
} catch (e) {
    console.log('FAILED:', e.code ?? e.name, '-', e.message, '| cause:', e.cause?.code, e.cause?.message);
    process.exitCode = 1;
} finally {
    server.close();
}

Run on Node 26:

pnpm --filter @qdrant/js-client-rest build
docker run --rm -v "$PWD":"$PWD" -w "$PWD/packages/js-client-rest" node:26-alpine node repro.mjs