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 90.35%. Comparing base (75e4bbe) to head (3833a92).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66161      +/-   ##
==========================================
- Coverage   90.36%   90.35%   -0.02%     
==========================================
  Files         792      792              
  Lines      275398   275430      +32     
  Branches    52776    52787      +11     
==========================================
- Hits       248877   248869       -8     
- Misses      16937    16984      +47     
+ Partials     9584     9577       -7     

Files with missing lines	Coverage Δ
src/node_zlib.cc	80.30% <100.00%> (+0.08%)	⬆️

... and 37 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/77689/

[ShogunPanda]

LGTM!

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/77875/

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