vfs: fix path returned by recursive mkdir

[watilde]

mkdir() with recursive: true returns the first directory that was created. VFS passed the provider's return value straight through, so the path came back in the wrong namespace: RealFSProvider returned an absolute path inside its root, leaking that root to the caller, and a mounted VFS returned a VFS-internal path without the mount point.

Map the created path back through the provider and mount translations so the returned value is a path the caller can hand to fs again. This covers the sync, callback, and promises forms.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.33%. Comparing base (dc7838c) to head (6f3bdfa).
⚠️ Report is 84 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65062      +/-   ##
==========================================
+ Coverage   90.30%   90.33%   +0.02%     
==========================================
  Files         759      760       +1     
  Lines      247629   248537     +908     
  Branches    46688    46903     +215     
==========================================
+ Hits       223618   224507     +889     
+ Misses      15469    15458      -11     
- Partials     8542     8572      +30     

Files with missing lines	Coverage Δ
lib/internal/vfs/file_system.js	99.37% <100.00%> (+<0.01%)	⬆️
lib/internal/vfs/providers/real.js	95.32% <100.00%> (+0.23%)	⬆️

... and 83 files with indirect coverage changes

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