fs: fix realpath of namespaced drive paths

[jazelly]

The JavaScript realpath implementation probes a namespaced drive root through the fs binding. Windows path resolution drops the trailing separator from that probe, so lstat receives C: and reports EISDIR.

Use the regular drive-root spelling only for the probe. Preserve the namespaced spelling for traversal and returned paths.

Fixes: #62446

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AI Disclaimer: I used AI to help me evaluate approaches. My initial attempt at #62639 was too breaking.

[codecov]

Codecov Report

❌ Patch coverage is 78.94737% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.12%. Comparing base (cf30b2e) to head (5750f39).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/fs.js	78.94%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65378      +/-   ##
==========================================
+ Coverage   90.11%   90.12%   +0.01%     
==========================================
  Files         752      752              
  Lines      251861   251876      +15     
  Branches    47365    47362       -3     
==========================================
+ Hits       226955   227015      +60     
+ Misses      16238    16187      -51     
- Partials     8668     8674       +6     

Files with missing lines	Coverage Δ
lib/fs.js	98.29% <78.94%> (-0.07%)	⬇️

... and 40 files with indirect coverage changes

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