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