vfs: fix rename over non-empty directory

[christianaurichzm]

MemoryProvider#renameSync() only rejected a destination whose type differed from the source (EISDIR/ENOTDIR). When both sides were directories it fell through to replacing the destination in its parent, so the destination directory and everything under it disappeared:

// node --experimental-vfs
const vfs = require('node:vfs').create();
vfs.mkdirSync('/src');
vfs.mkdirSync('/dst');
vfs.writeFileSync('/dst/keep.txt', 'keep');

vfs.renameSync('/src', '/dst');   // no error
vfs.existsSync('/dst/keep.txt');  // false

rename(2) accepts an existing directory as the destination only when it is empty, and fails with ENOTEMPTY otherwise. RealFSProvider delegates to fs.renameSync() and already behaves that way, so the two providers backing the same node:vfs API disagreed.

Two smaller fixes in the same function, both covered by the new tests:

• a file replaced by a rename now loses a link, so a hard link to it no longer reports an inflated nlink;
• a rename whose two names resolve to the same entry is a no-op; renaming one hard link onto another used to remove the source name while the file kept its old link count.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.07%. Comparing base (9baabd4) to head (e9333ce).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65613      +/-   ##
==========================================
- Coverage   90.07%   90.07%   -0.01%     
==========================================
  Files         751      751              
  Lines      254875   254928      +53     
  Branches    48108    48126      +18     
==========================================
+ Hits       229579   229624      +45     
- Misses      16466    16484      +18     
+ Partials     8830     8820      -10     

Files with missing lines	Coverage Δ
lib/internal/vfs/providers/memory.js	95.18% <100.00%> (+0.05%)	⬆️

... and 39 files with indirect coverage changes

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