fix: close kqueue fd in SetupExitCallback on macOS

[codebytere-ant]

Closes #907

On macOS, SetupExitCallback opens a kqueue() per spawned pty to wait on NOTE_EXIT, but never closes it before the watcher thread returns. This leaks one kqueue file descriptor per pty.spawn() for the lifetime of the host process.

Repro:

const pty = require('node-pty');
const { execSync } = require('child_process');
const kq = () => execSync(`lsof -p ${process.pid} | grep -c KQUEUE`, {encoding:'utf8'}).trim();
(async () => {
  console.log('baseline KQUEUE=' + kq());
  for (let i = 0; i < 20; i++) {
    const p = pty.spawn('/bin/sh', [], { name: 'xterm', cols: 80, rows: 24 });
    await new Promise(r => { p.onExit(r); setTimeout(() => p.kill(), 50); });
  }
  await new Promise(r => setTimeout(r, 500));
  console.log('after 20 cycles KQUEUE=' + kq());  // baseline + 20
})();

The Chromium reference this code is based on (kill_mac.cc) closes the kqueue via ScopedFD; the equivalent here is an explicit close(kq) once the kevent wait completes. close(-1) is a harmless EBADF if kqueue() itself failed, so no guard is needed.

Adds a regression test alongside the ptmx leak test from #882.

cc @deepak1556

[codebytere-ant]

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[deepak1556]

Thanks!