vfs: return FileHandle from fs.promises.open

[trivikr]

Fixes: #65729

Wrap mounted virtual file descriptors in the public FileHandle interface while delegating operations to the underlying provider handle.

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Assisted-by: codex:gpt-5.6-sol

[codecov]

Codecov Report

❌ Patch coverage is 87.80488% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.04%. Comparing base (f9ab994) to head (576a7a4).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/vfs/fd.js	87.17%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65730      +/-   ##
==========================================
- Coverage   90.07%   90.04%   -0.03%     
==========================================
  Files         754      754              
  Lines      256395   256433      +38     
  Branches    48494    48499       +5     
==========================================
- Hits       230947   230914      -33     
- Misses      16563    16624      +61     
- Partials     8885     8895      +10     

Files with missing lines	Coverage Δ
lib/internal/vfs/setup.js	87.32% <100.00%> (-0.02%)	⬇️
lib/internal/vfs/fd.js	96.03% <87.17%> (-1.67%)	⬇️

... and 32 files with indirect coverage changes

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