stream: wait for push writer end fallback to drain

[trivikr]

Fixes an issue in node:stream/iter where push() writer end() could
resolve before buffered data was consumed when called after endSync()
returned -1.

endSync() already transitions the queue to closing when data remains
buffered. A subsequent async end() now keeps waiting for that closing
state to drain, and reuses any existing pending end promise.

duplex() channel close now only signals EOF synchronously instead of
awaiting that drain, since the peer may not start reading until after
close() resolves.

Fixes: #63502

---

Assisted-by: openai:gpt-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

❌ Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.13%. Comparing base (8d3245e) to head (cee2067).

Files with missing lines	Patch %	Lines
lib/internal/streams/iter/push.js	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63503      +/-   ##
==========================================
- Coverage   90.14%   90.13%   -0.02%     
==========================================
  Files         718      718              
  Lines      227984   227987       +3     
  Branches    42835    42841       +6     
==========================================
- Hits       205522   205486      -36     
- Misses      14235    14284      +49     
+ Partials     8227     8217      -10     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/duplex.js	96.35% <100.00%> (-0.11%)	⬇️
lib/internal/streams/iter/push.js	92.01% <62.50%> (-0.20%)	⬇️

... and 28 files with indirect coverage changes

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