stream: avoid duplicate writes in toWritable

[trivikr]

toWritable() currently treats writeSync() / writevSync() returning
false as meaning the data was not accepted and retries through the async
writer method.

For PushWriter with backpressure: 'block', false can instead mean the
chunk was accepted and backpressure is active. Retrying through
write() / writev() writes the same data twice.

This updates the adapter to recognize that accepted-backpressure case and wait
for drain instead of retrying the chunk.

Fixes: #63359

---

Assisted-by: openai:gpt-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

❌ Patch coverage is 87.14286% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.04%. Comparing base (8c6e39e) to head (52ffb94).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/streams/iter/classic.js	80.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63360      +/-   ##
==========================================
- Coverage   90.05%   90.04%   -0.02%     
==========================================
  Files         714      714              
  Lines      225564   225628      +64     
  Branches    42660    42667       +7     
==========================================
+ Hits       203134   203168      +34     
- Misses      14209    14248      +39     
+ Partials     8221     8212       -9     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/push.js	92.20% <100.00%> (+0.12%)	⬆️
lib/internal/streams/iter/types.js	100.00% <100.00%> (ø)
lib/internal/streams/iter/classic.js	88.91% <80.00%> (-0.54%)	⬇️

... and 47 files with indirect coverage changes

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