fs: fix repeated copy of directory with symlinks

[haramj]

Fixes: #65097

Repeatedly copying a directory that contains a symlink to an unrelated
directory fails on the second copy with ERR_FS_CP_EINVAL because the
symlink target is mistaken for a self-referential copy.

Root cause

Both the JS (isSrcSubdir in lib/internal/fs/cp/cp.js) and the C++
(isInsideDir in src/node_file.cc) guards are prefix-inclusive, so when
the source symlink and the already-copied destination symlink resolve to
the same directory, the "copy to a subdirectory of self" check fires on
the identical target.

Fix

• Treat identical symlink targets as a self-copy only when the target is
  the destination root or one of its ancestors; identical targets that are
  unrelated to the destination root are allowed to be copied again.
• Compare targets using canonicalized paths (weakly_canonical) in C++
  so relative-vs-absolute representations and macOS /var vs
  /private/var are handled correctly.
• Thread the top-level destination root through the JS copy recursion via
  the internal options object.

Unlike the earlier attempt in #65099, the existing self-referential
protections are kept: a symlink pointing to the destination root or an
ancestor of it still fails with ERR_FS_CP_EINVAL.

Tests

New regression tests (test-fs-cp-sync-symlink-repeated-copy and
test-fs-cp-async-symlink-repeated-copy) cover both the C++ (no filter)
and JS (filter) paths with absolute and relative targets, each copied
twice, and verify the copied symlink still resolves to the original
target. Existing *-points-to-dest tests are unchanged and keep passing.

Validation

• make builds successfully.
• All test/parallel/test-fs-*.{mjs,js} (351 tests) pass, including the
  new regression tests and all *-points-to-dest tests.
• ESLint clean; git-clang-format reports no formatting changes.

[codecov]

Codecov Report

❌ Patch coverage is 92.68293% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.13%. Comparing base (d6bbf57) to head (7820495).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/fs/cp/cp-sync.js	83.33%	2 Missing ⚠️
src/node_file.cc	94.73%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65411      +/-   ##
==========================================
- Coverage   90.13%   90.13%   -0.01%     
==========================================
  Files         752      752              
  Lines      251864   252307     +443     
  Branches    47354    47464     +110     
==========================================
+ Hits       227021   227411     +390     
- Misses      16180    16202      +22     
- Partials     8663     8694      +31     

Files with missing lines	Coverage Δ
lib/internal/fs/cp/cp.js	90.07% <100.00%> (+1.72%)	⬆️
src/node_file.cc	74.01% <94.73%> (-0.26%)	⬇️
lib/internal/fs/cp/cp-sync.js	69.44% <83.33%> (+10.83%)	⬆️

... and 41 files with indirect coverage changes

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[haramj]

Hi @RafaelGSS @jasnell @Renegade334,

I've updated the PR to also cover the JavaScript (filter) path in the repeated-copy regression tests — cp-sync.js coverage for the new symlink-target logic is now complete. All CI checks pass.

Would you mind taking a look when you have a moment? Thanks!