stream: fix merge() idle source draining

[trivikr]

This updates stream/iter merge() so fast sources are not pulled to
completion while the merged consumer is idle.

Previously, after a source produced a batch, merge() immediately scheduled
another next() for that same source. If the merged iterator yielded once and
then the consumer paused, fast sources could continue draining in the
background.

The multi-source merge path now waits until the consumer requests the next
merged item before pulling again from the source that produced the previous
item.

Fixes: #63566

---

Assisted-by: openai:gpt-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

❌ Patch coverage is 89.28571% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.34%. Comparing base (49dcaa0) to head (1d1b87f).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/streams/iter/consumers.js	89.28%	12 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #63567    +/-   ##
========================================
  Coverage   90.33%   90.34%            
========================================
  Files         730      730            
  Lines      234362   234528   +166     
  Branches    43906    43924    +18     
========================================
+ Hits       211713   211879   +166     
- Misses      14371    14374     +3     
+ Partials     8278     8275     -3     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/consumers.js	96.48% <89.28%> (-0.14%)	⬇️

... and 30 files with indirect coverage changes

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