fs: fix fs.promises.realpath for long paths on Windows

Unlike other fs functions that work with paths, realpath isn't
using pathModule.toNamespacedPath prior to calling libuv function. This
is causing issues on windows.

Windows long path test is also improved to cover the mentioned issue.

Fixes: #51031
PR-URL: #51032
Reviewed-By: LiviaMedeiros <livia@cirno.name>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/fs/promises.js

@@ -1010,7 +1010,7 @@ async function lutimes(path, atime, mtime) {
 async function realpath(path, options) {
   options = getOptions(options);
   path = getValidatedPath(path);
-  return binding.realpath(path, options.encoding, kUsePromises);
+  return binding.realpath(pathModule.toNamespacedPath(path), options.encoding, kUsePromises);
 }
 
 async function mkdtemp(prefix, options) {

test/parallel/test-fs-long-path.js

@@ -46,4 +46,7 @@ fs.writeFile(fullPath, 'ok', common.mustSucceed(() => {
 
   // Tests https://github.com/nodejs/node/issues/39721
   fs.realpath.native(fullPath, common.mustSucceed());
+
+  // Tests https://github.com/nodejs/node/issues/51031
+  fs.promises.realpath(fullPath).then(common.mustCall(), common.mustNotCall());
 }));