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 90.13%. Comparing base (2f56cd2) to head (4236470).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #63358 +/- ##
==========================================
- Coverage 90.17% 90.13% -0.05% 
==========================================
Files 718 718 Lines 227731 227923 +192 Branches 42768 42828 +60 ==========================================
+ Hits 205365 205430 +65 - Misses 14145 14277 +132 + Partials 8221 8216 -5 

Files with missing lines	Coverage Δ
lib/internal/streams/iter/broadcast.js	85.23% <100.00%> (+0.70%)	⬆️
lib/internal/streams/iter/share.js	85.84% <100.00%> (+1.72%)	⬆️

... and 39 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/73617/

[nodejs-github-bot]

Landed in a7d5446