fs: fix cp symlink and EEXIST handling on Windows

[PickBas]

fs: fix cp symlink and EEXIST handling on Windows

On Windows, fs.cp/fs.cpSync had two bugs:

• Paths from std::filesystem carried the \\?\ extended-length prefix, breaking path comparison and symlink targets. Now stripped.
• The EEXIST check used the raw POSIX value, which never matched on Windows. Now compares against std::errc::file_exists.

Re-enables 2 tests that were skipped for this issue.

Fixes: #59636

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.24%. Comparing base (e6a8d06) to head (33b9810).
⚠️ Report is 287 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_file.cc	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64353      +/-   ##
==========================================
- Coverage   92.01%   90.24%   -1.77%     
==========================================
  Files         379      741     +362     
  Lines      166972   240984   +74012     
  Branches    25554    45398   +19844     
==========================================
+ Hits       153639   217476   +63837     
- Misses      13041    15069    +2028     
- Partials      292     8439    +8147     

Files with missing lines	Coverage Δ
src/node_file.cc	74.15% <0.00%> (ø)

... and 545 files with indirect coverage changes

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