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 90.32%. Comparing base (8b88e2c) to head (37e6bae).

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   91.95%   90.32%   -1.64%     
==========================================
  Files         379      732     +353     
  Lines      166486   236519   +70033     
  Branches    25452    44552   +19100     
==========================================
+ Hits       153099   213644   +60545     
- Misses      13094    14604    +1510     
- Partials      293     8271    +7978     

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

... and 480 files with indirect coverage changes

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