fs: support Buffer paths in cp methods

[StealthEyeLLC]

Fixes: #58634

fs.cp(), fs.promises.cp(), and fs.cpSync() accept path-like arguments, but the recursive copy implementation used string-only path operations and decoded directory entry names as UTF-8. As a result, Buffer paths failed during recursive copies and non-UTF-8 file names could not be preserved.

This change adds internal path helpers that operate on strings and Buffers. Recursive directory traversal reads entry names with encoding: 'buffer' and retains Buffer paths where conversion to a string would lose information.

The existing cpSync() native fast path is still used when both paths are strings and no filter is present. Buffer paths use the JavaScript traversal.

Filter callback arguments remain strings for string and file URL inputs when the paths can be represented as strings. Buffer arguments are used when the corresponding paths need to remain byte-preserving. The fs documentation has been updated accordingly.

Tests cover:

• synchronous, callback, and promise APIs
• recursive copies and filters
• string, Buffer, URL, and mixed path types
• nested non-UTF-8 file names on Linux
• symlinks and self-copy checks
• internal string and Buffer path helpers

The previous known-issue tests for these cases have been removed because the cases now pass.

Validation:

• make -j4 test
• 5,770 tests passed
• 0 tests failed
• tested source tree: 5ad46ea90e50e226fec7b35523786ccc72a16c53
• test log SHA-256: 52b492c326feb4b9e000316a775b6924b71ffcd34b177b25df7e5ffe0999fa47