stream: improve handling of falsy errors in stream/iter

[jasnell]

• Preserves arbitrary failure and cancellation reasons, including falsy values such as undefined, null, false, 0, and ''.
• Distinguishes omitted cancellation reasons from explicitly passed undefined.
• Removes internal coercion of non-Error reasons.
• Wraps non-Error values only at classic stream boundaries, retaining the original value in error.reason.
• Improves pending write/end cleanup for file, QUIC, broadcast, push, share, and classic stream writers.
• Ensures terminal writer errors take precedence over per-operation abort signals.
• Adds extensive reason-propagation and lifecycle tests.
• Updates stream/iter documentation accordingly.

[nodejs-github-bot]

Review requested:

• @nodejs/quic
• @nodejs/streams

[codecov]

Codecov Report

❌ Patch coverage is 80.14888% with 80 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.20%. Comparing base (8af7545) to head (fdbfc17).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/fs/promises.js	72.78%	39 Missing and 1 partial ⚠️
lib/internal/streams/iter/classic.js	80.27%	25 Missing and 4 partials ⚠️
lib/internal/streams/iter/push.js	81.48%	5 Missing ⚠️
lib/internal/streams/iter/share.js	92.68%	3 Missing ⚠️
lib/internal/streams/iter/broadcast.js	91.66%	2 Missing ⚠️
lib/internal/streams/iter/transform.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65864      +/-   ##
==========================================
+ Coverage   90.18%   90.20%   +0.02%     
==========================================
  Files         771      771              
  Lines      264619   264786     +167     
  Branches    50231    50286      +55     
==========================================
+ Hits       238653   238862     +209     
+ Misses      16961    16923      -38     
+ Partials     9005     9001       -4     

Files with missing lines	Coverage Δ
lib/internal/abort_controller.js	95.35% <100.00%> (+<0.01%)	⬆️
lib/internal/quic/quic.js	100.00% <100.00%> (ø)
lib/internal/streams/iter/pull.js	83.75% <100.00%> (+0.23%)	⬆️
lib/internal/streams/iter/utils.js	98.39% <ø> (-0.05%)	⬇️
lib/internal/streams/iter/transform.js	95.83% <0.00%> (+0.11%)	⬆️
lib/internal/streams/iter/broadcast.js	90.19% <91.66%> (+0.70%)	⬆️
lib/internal/streams/iter/share.js	87.80% <92.68%> (+0.34%)	⬆️
lib/internal/streams/iter/push.js	92.96% <81.48%> (+<0.01%)	⬆️
lib/internal/streams/iter/classic.js	88.96% <80.27%> (-0.26%)	⬇️
lib/internal/fs/promises.js	90.82% <72.78%> (-0.29%)	⬇️

... and 27 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/77118/