stream: avoid draining merged iter sources

[trivikr]

Fixes: #63566

stream/iter merge() was scheduling the next read from a source as soon as
that source produced a value. For fast sources, this could drain all inputs even
while the merged consumer was idle after a single next().

This defers each source's next read until the merged iterator resumes after
yielding the previous value.

---

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 90.25%. Comparing base (799e910) to head (0b25aaa).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #64293   +/-   ##
=======================================
  Coverage   90.25%   90.25%           
=======================================
  Files         741      741           
  Lines      241165   241167    +2     
  Branches    45428    45419    -9     
=======================================
+ Hits       217667   217669    +2     
  Misses      15069    15069           
  Partials     8429     8429           

Files with missing lines	Coverage Δ
lib/internal/streams/iter/consumers.js	97.15% <100.00%> (+0.68%)	⬆️

... and 26 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74677/

[nodejs-github-bot]

Landed in 0032189