vfs: fix promises.watch() with an already-aborted signal

[trivikr]

Fixes: #66253

fs.promises.watch() over a virtual mount resolved the first next() with { done: true } when the signal was already aborted, while native fs.promises.watch() rejects with ABORT_ERR.

Buffer the AbortError in VFSWatchAsyncIterable so the first next() rejects once and later calls behave like an exhausted iterator, matching the native async generator. Concurrently pending next() calls now resolve after the rejecting one instead of all rejecting.

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Assisted-by: opencode

[codecov]

Codecov Report

❌ Patch coverage is 76.47059% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.28%. Comparing base (a2c8da5) to head (ebc1f19).

Files with missing lines	Patch %	Lines
lib/internal/vfs/watcher.js	76.47%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #66254   +/-   ##
=======================================
  Coverage   90.28%   90.28%           
=======================================
  Files         789      789           
  Lines      272878   272893   +15     
  Branches    52097    52102    +5     
=======================================
+ Hits       246360   246380   +20     
+ Misses      16975    16973    -2     
+ Partials     9543     9540    -3     

Files with missing lines	Coverage Δ
lib/internal/vfs/watcher.js	94.74% <76.47%> (-0.47%)	⬇️

... and 25 files with indirect coverage changes

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