win: fix fs.promises.realpath for long paths

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

lib/internal/fs/promises.js

@@ -1160,7 +1160,7 @@ async function realpath(path, options) {
   options = getOptions(options);
   path = getValidatedPath(path);
   return await PromisePrototypeThen(
-    binding.realpath(path, options.encoding, kUsePromises),
+    binding.realpath(pathModule.toNamespacedPath(path), options.encoding, kUsePromises),
     undefined,
     handleErrorFromBinding,
   );

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());
 }));