stream: honor AbortSignal in Writer.end()

[trivikr]

Fixes: #64726

This change makes end() reject with signal.reason when passed a
pre-aborted signal, without closing the writer. For push writers, aborting
the signal while buffered data is draining rejects that end() operation
while allowing the underlying graceful close to continue.

---

Assisted-by: codex:gpt-5.6-sol

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

❌ Patch coverage is 92.10526% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.15%. Comparing base (b420cb6) to head (849b419).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/streams/iter/push.js	91.42%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64727      +/-   ##
==========================================
+ Coverage   90.13%   90.15%   +0.01%     
==========================================
  Files         743      743              
  Lines      242412   242440      +28     
  Branches    45655    45660       +5     
==========================================
+ Hits       218510   218568      +58     
+ Misses      15403    15391      -12     
+ Partials     8499     8481      -18     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/broadcast.js	87.75% <100.00%> (+0.01%)	⬆️
lib/internal/streams/iter/push.js	91.42% <91.42%> (+0.50%)	⬆️

... and 37 files with indirect coverage changes

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