zlib: avoid an extra empty zstd frame

[marcopiraccini]

createZstdCompress() could append an empty frame when end() was called while writes were still queued. The last queued chunk could finish the frame, then the stream's final flush sent ZSTD_e_end again with no input.

Track when the frame has ended and skip empty calls until more input arrives. This avoids the extra frame while still allowing a later write to start a new one.

Added test/parallel/test-zlib-zstd-compress-single-frame.js to cover queued writes, explicit frame endings, an empty flush, an empty stream, and a later frame.

Fixes: #66078

Note

PR #66091 proposed the same C++ fix but was closed without merging. This PR expands the regression coverage to include an empty flush after a completed frame, an empty stream, and a deliberate second frame, alongside queued writes and an explicit end flush. The empty flush completes before end() so the test exercises ZSTD_e_flush separately.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.28%. Comparing base (25e6c15) to head (b0ce1f4).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66161      +/-   ##
==========================================
+ Coverage   90.27%   90.28%   +0.01%     
==========================================
  Files         790      790              
  Lines      271981   271988       +7     
  Branches    51913    51920       +7     
==========================================
+ Hits       245531   245568      +37     
+ Misses      16945    16912      -33     
- Partials     9505     9508       +3     

Files with missing lines	Coverage Δ
src/node_zlib.cc	79.88% <100.00%> (-0.23%)	⬇️

... and 35 files with indirect coverage changes

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