vfs: address review comments on cp-sync and test docs

Use optional chaining for vfsState.handlers checks in cp-sync.js
and reuse stat result to avoid double statSync call. Add YAML
metadata to MockFSContext class documentation.

Author: mcollina

doc/api/test.md

@@ -2416,6 +2416,10 @@ The mock file system is automatically cleaned up when the test completes.
 
 ## Class: `MockFSContext`
 
+<!-- YAML
+added: REPLACEME
+-->
+
 The `MockFSContext` object is returned by `mock.fs()` and provides the
 following methods and properties:
 

lib/internal/fs/cp/cp-sync.js

@@ -56,7 +56,7 @@ function cpSyncFn(src, dest, opts) {
   // Skip C++ path checks for VFS paths (C++ lstat can't see VFS files).
   // The JS-level getStats() calls lstatSync which is VFS-intercepted.
   const { vfsState } = require('internal/fs/utils');
-  if (vfsState.handlers === null || !vfsState.handlers.statSync(src)) {
+  if (!vfsState.handlers?.statSync(src)) {
     fsBinding.cpSyncCheckPaths(src, dest, opts.dereference, opts.recursive);
   } else {
     // Minimal JS-level validation that mirrors what C++ does
@@ -103,7 +103,7 @@ function onFile(srcStat, destStat, src, dest, opts) {
   if (opts.force) {
     // Skip C++ fast path for VFS paths (C++ can't see VFS files).
     const { vfsState } = require('internal/fs/utils');
-    if (vfsState.handlers === null || !vfsState.handlers.statSync(src)) {
+    if (!vfsState.handlers?.statSync(src)) {
       return fsBinding.cpSyncOverrideFile(src, dest, opts.mode, opts.preserveTimestamps);
     }
     unlinkSync(dest);
@@ -167,7 +167,7 @@ function copyDir(src, dest, opts, mkDir, srcMode) {
     // we can run the whole function faster in C++.
     // Skip C++ fast path for VFS paths (C++ can't see VFS files).
     const { vfsState } = require('internal/fs/utils');
-    if (vfsState.handlers === null || !vfsState.handlers.statSync(src)) {
+    if (!vfsState.handlers?.statSync(src)) {
       // TODO(dario-piotrowicz): look into making cpSyncCopyDir also accept the potential filter function
       return fsBinding.cpSyncCopyDir(src, dest,
                                      opts.force,
@@ -240,8 +240,9 @@ function onLink(destStat, src, dest, verbatimSymlinks) {
   // internalModuleStat is a C++ call that can't see VFS files.
   const { vfsState } = require('internal/fs/utils');
   let srcIsDir;
-  if (vfsState.handlers !== null && vfsState.handlers.statSync(src)) {
-    srcIsDir = statSync(src).isDirectory();
+  const vfsStat = vfsState.handlers?.statSync(src);
+  if (vfsStat) {
+    srcIsDir = vfsStat.isDirectory();
   } else {
     srcIsDir = fsBinding.internalModuleStat(src) === 1;
   }