lib: fix readdir in recursive mode with a buffer argument

[y-okt]

Fixes: #58892

Calling readdir with a Buffer argument and the recursive flag produces an ERR_INVALID_ARG_TYPE error. The expected behavior would be for the readdir functions not to error and return either an array of Buffers or Dirents (depending on options.withFileTypes)

[codecov]

Codecov Report

❌ Patch coverage is 96.61017% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.54%. Comparing base (4f24aff) to head (909880c).

Files with missing lines	Patch %	Lines
lib/internal/fs/promises.js	93.10%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60936      +/-   ##
==========================================
+ Coverage   88.53%   88.54%   +0.01%     
==========================================
  Files         703      703              
  Lines      208546   208596      +50     
  Branches    40217    40238      +21     
==========================================
+ Hits       184634   184701      +67     
+ Misses      15926    15905      -21     
- Partials     7986     7990       +4     

Files with missing lines	Coverage Δ
lib/fs.js	98.20% <100.00%> (+0.01%)	⬆️
lib/internal/fs/promises.js	98.03% <93.10%> (-0.12%)	⬇️

... and 38 files with indirect coverage changes

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[Mohataseem89]

correct handling of buffer arguments in recursive readdir aligns the behavior with people's expectations and makes the fs API more consistent

the Codecov report shows two uncovered lines in promises.js - adding a test for the recursive + Buffer path would close that gap and make the fix fully covered