vfs: reject statfs for missing paths

[trivikr]

Fixes: #65692

Validate mounted VFS paths before returning synthetic statfs data so fs.statfsSync() and fs.promises.statfs() report ENOENT for missing paths. Forward validation errors asynchronously to fs.statfs() callbacks.

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Assisted-by: codex:gpt-5.6-sol

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.06%. Comparing base (242909f) to head (859f592).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65693      +/-   ##
==========================================
- Coverage   90.06%   90.06%   -0.01%     
==========================================
  Files         754      754              
  Lines      256395   256402       +7     
  Branches    48500    48504       +4     
==========================================
+ Hits       230922   230927       +5     
- Misses      16574    16580       +6     
+ Partials     8899     8895       -4     

Files with missing lines	Coverage Δ
lib/fs.js	98.43% <100.00%> (+<0.01%)	⬆️
lib/internal/vfs/setup.js	87.35% <100.00%> (+0.01%)	⬆️

... and 26 files with indirect coverage changes

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