fs: fix mkdtempDisposable with Buffer paths

[gengjiawen]

fs.mkdtempDisposableSync() and fsPromises.mkdtempDisposable() resolve the path of the new directory with path.resolve(), so that remove() still works after process.chdir(). path.resolve() only accepts strings, so when the path is a Buffer (a Buffer prefix, or encoding: 'buffer'), both functions throw ERR_INVALID_ARG_TYPE after the directory has been created, and the directory is never removed.

absoluteMkdtempPath() in internal/fs/utils stashes that path for remove():

• string paths are resolved exactly as before
• on Windows, a Buffer path is UTF-8, so it is resolved with path.resolve() and encoded back. Root-relative (\foo) and drive-relative (D:foo) prefixes keep win32 semantics, and non-ASCII characters in the cwd stay intact
• on POSIX, an absolute Buffer is kept byte for byte, including bytes that are not valid UTF-8. A relative Buffer gets the cwd from creation time prepended, via the existing join() used for other string-plus-Buffer paths

The tests cover both the sync and the promise API: a Buffer prefix, encoding: 'buffer', a relative Buffer prefix followed by process.chdir(), a relative Buffer prefix in a non-ASCII directory, on Linux a prefix containing a byte that is not valid UTF-8, and on POSIX a .. component after a symlink, so remove() deletes the directory the OS actually created.

Fixes: #66593
Refs: #64397

AI disclosure: written with the help of Claude (claude:opus-5.5) and Grok (grok-4.7). I checked the bug against the source and the latest nightly, and reviewed the fix and tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.44%. Comparing base (6d5e309) to head (f88356b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66594      +/-   ##
==========================================
- Coverage   90.44%   90.44%   -0.01%     
==========================================
  Files         791      791              
  Lines      276562   276592      +30     
  Branches    53126    53125       -1     
==========================================
+ Hits       250130   250154      +24     
+ Misses      16846    16841       -5     
- Partials     9586     9597      +11     

Files with missing lines	Coverage Δ
lib/fs.js	97.32% <100.00%> (+<0.01%)	⬆️
lib/internal/fs/promises.js	91.15% <100.00%> (+0.13%)	⬆️
lib/internal/fs/utils.js	96.37% <100.00%> (+0.08%)	⬆️

... and 27 files with indirect coverage changes

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