stream: fix overlapping broadcast next calls

[trivikr]

broadcast() consumers could leave an earlier next() call pending forever
when another next() was called before data became available. The later call
overwrote the stored resolver, so the next write resolved the newer promise
instead of the older one.

This updates broadcast consumers to keep the first pending read intact. A
later overlapping next() now closes that consumer, while the original pending
read still receives the next chunk.

Fixes: #63499

---

Assisted-by: openai:gpt-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.15%. Comparing base (8d3245e) to head (87668cb).

Files with missing lines	Patch %	Lines
lib/internal/streams/iter/broadcast.js	72.72%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #63500   +/-   ##
=======================================
  Coverage   90.14%   90.15%           
=======================================
  Files         718      718           
  Lines      227984   227995   +11     
  Branches    42835    42832    -3     
=======================================
+ Hits       205522   205550   +28     
+ Misses      14235    14232    -3     
+ Partials     8227     8213   -14     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/broadcast.js	85.06% <72.72%> (-0.17%)	⬇️

... and 31 files with indirect coverage changes

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