vfs: normalize RealFSProvider mkdir return paths

[trivikr]

Fixes: #66220

RealFSProvider.mkdirSync()/mkdir() forwarded the raw fs.mkdirSync() result, which for { recursive: true } is the absolute backing-directory path of the first created directory. VirtualFileSystem#toMountedPath() then joined the mount point with it, producing <mount>/<root>/a instead of <mount>/a, so the returned path failed fs.existsSync().

Reuse the existing #resolvedToVfsPath() helper (as realpath does) to translate the result back to a provider-relative VFS path, matching MemoryProvider.

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Assisted-by: opencode

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.36%. Comparing base (c0ed28f) to head (3b52939).
⚠️ Report is 139 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66221      +/-   ##
==========================================
+ Coverage   90.29%   90.36%   +0.07%     
==========================================
  Files         790      792       +2     
  Lines      272883   275396    +2513     
  Branches    52118    52781     +663     
==========================================
+ Hits       246387   248852    +2465     
- Misses      16943    16974      +31     
- Partials     9553     9570      +17     

Files with missing lines	Coverage Δ
lib/internal/vfs/providers/real.js	95.75% <100.00%> (+0.06%)	⬆️

... and 141 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/78005/