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.

---

Assisted-by: codex:gpt-5.6-sol

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.05%. Comparing base (f9ab994) to head (40bd7ae).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65693      +/-   ##
==========================================
- Coverage   90.07%   90.05%   -0.03%     
==========================================
  Files         754      754              
  Lines      256395   256402       +7     
  Branches    48494    48506      +12     
==========================================
- Hits       230947   230893      -54     
- Misses      16563    16622      +59     
- Partials     8885     8887       +2     

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 31 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.