module: exclude node:vfs from builtinModules when flag is disabled

Mirrors the recently-merged handling for node:ffi (see #63158): when
--experimental-vfs is not set, node:vfs is filtered out of the public
Module.builtinModules list. Adds matching coverage in test-vfs-flag.

Refs: #63158

Assisted-by: Claude-Opus4.7
Signed-off-by: Matteo Collina <hello@matteocollina.com>

Author: mcollina

lib/internal/modules/cjs/loader.js

@@ -490,6 +490,9 @@ function initializeCJS() {
   if (!getOptionValue('--experimental-quic')) {
     modules = modules.filter((i) => i !== 'node:quic');
   }
+  if (!getOptionValue('--experimental-vfs')) {
+    modules = modules.filter((i) => i !== 'node:vfs');
+  }
   Module.builtinModules = ObjectFreeze(modules);
 
   initializeCjsConditions();

test/parallel/test-vfs-flag.js

@@ -48,3 +48,19 @@ const assert = require('assert');
   assert.strictEqual(r.status, 1);
   assert.match(r.stderr.toString(), /Cannot find module 'vfs'/);
 }
+
+// Module.builtinModules reflects whether --experimental-vfs is active.
+for (const [flag, expected] of [
+  ['--experimental-vfs', 'true\n'],
+  ['--no-experimental-vfs', 'false\n'],
+]) {
+  const r = spawnSync(process.execPath, [
+    flag,
+    '-p',
+    'require("node:module").builtinModules.includes("node:vfs")',
+  ], { encoding: 'utf8' });
+  assert.strictEqual(r.stdout, expected);
+  assert.strictEqual(r.stderr, '');
+  assert.strictEqual(r.status, 0);
+  assert.strictEqual(r.signal, null);
+}