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 90.16%. Comparing base (b4c83f5) to head (a94343b).
⚠️ Report is 11 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   90.16%   90.16%   -0.01%     
==========================================
  Files         771      771              
  Lines      265094   265132      +38     
  Branches    50367    50365       -2     
==========================================
+ Hits       239035   239057      +22     
- Misses      17000    17015      +15     
- Partials     9059     9060       +1     

Files with missing lines	Coverage Δ
lib/internal/vfs/setup.js	87.78% <100.00%> (-0.02%)	⬇️
lib/internal/vfs/fd.js	96.03% <87.17%> (-1.67%)	⬇️

... and 17 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/77169/