vfs: write RealFSProvider files to open fd

[christianaurichzm]

RealFileHandle.writeFile() and writeFileSync() reopen the original real path instead of writing through the handle's file descriptor. A write after the backing file is renamed can recreate the old path, and a read-only handle can write because the path is reopened with the default w flag.

// Flags: --experimental-vfs
const provider = new vfs.RealFSProvider(root);

const ro = await provider.open('/a.txt', 'r');
await ro.writeFile('CLOBBERED');

const rw = await provider.open('/b.txt', 'r+');
fs.renameSync(path.join(root, 'b.txt'), path.join(root, 'c.txt'));
await rw.writeFile('NEW');

On main:

a.txt: CLOBBERED
c.txt: bbb
b.txt: NEW

Write through the open fd instead, mirroring #64104 for reads. The async path keeps iterable input support and follows filehandle.writeFile() current-position semantics, which is what #65854 is applying to the memory and zip handles. flush remains one fsync per call.

Tests cover writes after rename, read-only handles, iterable input, option and abort handling, and flush. Two existing assertions encoded the replacing behaviour and are updated.

On linux-x64, make lint, all 115 test-vfs-* tests, and the configured parallel and sequential suites pass.

Refs: #64104
Refs: #64103
Refs: #65854

[codecov]

Codecov Report

❌ Patch coverage is 98.87640% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.17%. Comparing base (b113d09) to head (7865323).
⚠️ Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/vfs/providers/real.js	98.87%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #65885   +/-   ##
=======================================
  Coverage   90.16%   90.17%           
=======================================
  Files         771      771           
  Lines      265094   265172   +78     
  Branches    50355    50383   +28     
=======================================
+ Hits       239027   239111   +84     
+ Misses      17011    17006    -5     
+ Partials     9056     9055    -1     

Files with missing lines	Coverage Δ
lib/internal/vfs/providers/real.js	95.53% <98.87%> (+0.43%)	⬆️

... and 23 files with indirect coverage changes

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[meixg]

nit: isCustomIterable() and checkAborted() already exist in lib/internal/fs/promises.js. Worth considering whether we can reuse them.

[christianaurichzm]

Thanks. I looked into this. Both helpers are currently module-local in internal/fs/promises.js, so reusing them would require exporting them or moving them to a shared utility. checkAborted also already has a separate module-local implementation in lib/fs.js. I kept the small copies local to avoid broadening this fix, but I'm happy to extract them if you prefer.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/77177/