stream: serialize concurrent share consumer reads

[trivikr]

This updates stream/iter share() consumers to serialize overlapping
next() calls from the same async iterator, preserving call order when
multiple reads are started before earlier reads resolve.

A regression test was added for the reported case where two concurrent
next() calls returned second before first.

Fixes: #63477

---

Assisted-by: openai:gpt-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

❌ Patch coverage is 81.35593% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.13%. Comparing base (8d3245e) to head (1d297dc).

Files with missing lines	Patch %	Lines
lib/internal/streams/iter/share.js	81.35%	10 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63478      +/-   ##
==========================================
- Coverage   90.14%   90.13%   -0.01%     
==========================================
  Files         718      718              
  Lines      227984   227995      +11     
  Branches    42835    42829       -6     
==========================================
- Hits       205522   205513       -9     
- Misses      14235    14253      +18     
- Partials     8227     8229       +2     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/share.js	84.19% <81.35%> (-1.66%)	⬇️

... and 29 files with indirect coverage changes

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