http2: finish pending writes after stream close

[trivikr]

This fixes a race where an HTTP/2 stream can close while writes are still
queued. In that state, pending write callbacks could be left unresolved, which
kept the test process alive until timeout.

The fix completes pending write callbacks when nghttp2 closes the stream and
prevents additional data from being queued once the stream is closed.

The existing regression test now also verifies that all server-side write
callbacks run before the stream closes.

Fixes: #58252

Tested with:

• python3 tools/test.py parallel/test-http2-close-while-writing
• python3 tools/test.py --repeat=1000 parallel/test-http2-close-while-writing
• python3 tools/test.py --repeat=10000 parallel/test-http2-close-while-writing

---

Assisted-by: openai:gpt-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/http2
• @nodejs/net

[lpinca]

See also #63249

As a happy side bonus, as part of this work I hunted down the underlying issue for #58252: buggy cleanup of reset streams.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.06%. Comparing base (0b5b189) to head (58e9183).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_http2.cc	84.61%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #63439   +/-   ##
=======================================
  Coverage   90.06%   90.06%           
=======================================
  Files         714      714           
  Lines      225918   225931   +13     
  Branches    42734    42739    +5     
=======================================
+ Hits       203464   203480   +16     
- Misses      14232    14233    +1     
+ Partials     8222     8218    -4     

Files with missing lines	Coverage Δ
lib/internal/http2/core.js	95.19% <100.00%> (+<0.01%)	⬆️
src/node_http2.h	91.71% <ø> (+0.09%)	⬆️
src/node_http2.cc	82.09% <84.61%> (+<0.01%)	⬆️

... and 29 files with indirect coverage changes

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