Fix promise transfer settlement stalling on node 26.4+

[maksimyaromin]

Problem

When a promise crosses isolates with { promise: true }, the other side receives its own promise which is settled later by TransferablePromiseHolder::ResolveTask. In the nodejs isolate, microtasks run under the explicit policy, so the settlement only queues the await continuations. No checkpoint runs afterwards, and they sit in the queue until some other JS callback triggers one. In a process that's just holding a server or an IPC channel, that's never.

Why node 26.4

This stayed invisible because node used to run a checkpoint on every loop iteration, as a side effect of draining native immediates even when there was nothing to drain. nodejs/node#62969 (26.4.0) skips the empty drain, and the accidental checkpoint went with it.

Repro

// main.mjs, hangs on node >= 26.4
import ivm from 'isolated-vm';
import net from 'node:net';

net.createServer().listen(0); // just something to keep the process alive

const isolate = new ivm.Isolate();
const context = await isolate.createContext();
await context.evalClosure(
	'sleep = ms => $0.applySync(undefined, [ ms ], { result: { promise: true } })',
	[ ms => new Promise(resolve => setTimeout(resolve, ms)) ],
	{ arguments: { reference: true } });

await context.eval('sleep(5)', { promise: true });
console.log('settled');
process.exit();

Fix

The fix runs the checkpoint in ResolveTask::Run once the promise settles. Also adds a regression test, which fails on 26.4+ without the fix.