vfs: route mkdtempDisposable through the fs hooks - #66695

fs.mkdtempDisposableSync() and fsPromises.mkdtempDisposable() bypass the VFS handlers that fs.mkdtempSync() and fsPromises.mkdtemp() consult, so on a mounted path they fail with ENOTDIR (the mount point does not exist on the real file system):

const myVfs = vfs.create();
myVfs.mkdirSync('/work', { recursive: true });
const prefix = `${myVfs.mount()}/work/tmp-`;
fs.mkdtempSync(prefix);                 // ok
fs.mkdtempDisposableSync(prefix);       // ENOTDIR

This adds the two handlers, built on the VFS mkdtemp and rm, and a test covering the sync and promise variants, Symbol.dispose and Symbol.asyncDispose, the buffer encoding, repeated remove() calls, and a prefix outside the mount.

Verified locally on Linux: the new test, every test/parallel/test-vfs-* test and the fs mkdtemp tests pass. Same approach as #65852.

A coding assistant helped with the investigation and the first draft; I reviewed the change, built Node and ran the tests locally.