fs: support Buffer paths in cp() and cpSync()

[HuzaifaAbdulRehman]

What breaks

fs.cpSync() with a filter, and fs.promises.cp(), throw ERR_INVALID_ARG_TYPE when src or dest is a Buffer, though Buffer paths are accepted elsewhere in fs (they address non-UTF-8 byte file names on POSIX).

const { cpSync, mkdirSync } = require('node:fs');
mkdirSync('a/c', { recursive: true });
cpSync(Buffer.from('a'), Buffer.from('b'), { recursive: true, filter: () => true });
// TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string.
//     at join (node:path)
//     at copyDir (node:internal/fs/cp/cp-sync)

Root cause

The recursive walk builds child paths with path.join(src, name), and the async path also runs path.resolve() / path.dirname() in its structural checks. All reject a Buffer. cpSync without a filter avoids it (that branch runs in C++); cp.promises hits it in isSrcSubdir before copying.

The change

joinPath joins entries onto Buffer paths by concatenating bytes, and copyDir reads entries with encoding: 'buffer' for Buffer sources, so non-UTF-8 names survive rather than being mangled. toPathString decodes to a string only for the subdirectory and parent-directory checks; the copy keeps the Buffers.

String paths over a non-UTF-8-named entry stay unchanged (a string cannot carry those bytes) and remain a known issue, nearer #58869.

Tests

Promotes the two #58634 known-issue tests into test/parallel/ and adds a Linux-only byte-fidelity test (a Shift-JIS file name copied intact through Buffer paths).

Fixes: #58634

[codecov]

Codecov Report

❌ Patch coverage is 87.71930% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.04%. Comparing base (e68a93a) to head (cb6e436).
⚠️ Report is 23 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/fs/cp/cp.js	85.71%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65713      +/-   ##
==========================================
- Coverage   90.06%   90.04%   -0.02%     
==========================================
  Files         754      754              
  Lines      256360   256438      +78     
  Branches    48486    48508      +22     
==========================================
+ Hits       230891   230919      +28     
- Misses      16584    16629      +45     
- Partials     8885     8890       +5     

Files with missing lines	Coverage Δ
lib/internal/fs/cp/cp-sync.js	70.91% <100.00%> (+0.35%)	⬆️
lib/internal/fs/cp/cp.js	88.88% <85.71%> (-0.14%)	⬇️

... and 43 files with indirect coverage changes

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