stream: propagate abort reason in share and broadcast

[trivikr]

This updates broadcast({ signal }) and share(source, { signal }) to
propagate signal.reason when the signal aborts.

Previously, the abort handlers called cancel() with no reason. That
made waiting consumers observe clean iterator completion instead of
rejecting with the signal's abort reason, which is an AbortError for
AbortController.abort().

The Iterable Streams draft defines BroadcastOptions.signal and ShareOptions.signal,
and defines Broadcast.cancel(reason) such that consumers see an error when a reason
is provided and clean completion otherwise:

• https://stream-iter.jasnell.me/#dictdef-broadcastoptions
• https://stream-iter.jasnell.me/#dictdef-shareoptions
• https://stream-iter.jasnell.me/#broadcast-cancel

This change aligns broadcast() and share() with that behavior by calling
cancel(signal.reason) on abort. It also ensures pending share() consumers
that resume after cancellation rethrow the stored source error instead of
converting it back into clean completion.

Fixes: #63357

---

Assisted-by: openai:gpt-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.06%. Comparing base (c0327d3) to head (bf6a9c3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63358      +/-   ##
==========================================
- Coverage   90.08%   90.06%   -0.03%     
==========================================
  Files         714      714              
  Lines      225516   225517       +1     
  Branches    42655    42646       -9     
==========================================
- Hits       203161   203104      -57     
- Misses      14136    14183      +47     
- Partials     8219     8230      +11     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/broadcast.js	85.32% <100.00%> (+0.65%)	⬆️
lib/internal/streams/iter/share.js	85.77% <100.00%> (+1.64%)	⬆️

... and 28 files with indirect coverage changes

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