zlib: restart format detection when resetting Unzip

[marcopiraccini]

createUnzip().reset() left format detection state from the previous input. After a partial gzip header, the saved header-byte count was not cleared. After detecting gzip or zlib-wrapped deflate, the stream no longer used UNZIP
mode to detect the next input's format.

PR #64620 proposed a similar fix and was closed without merging. This patch tracks whether the stream was created as UNZIP and separates public reset from the internal gzip-member reset. It also tests resetting from detected gzip to deflate.

Fixes: #64619

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.31%. Comparing base (ea08334) to head (389d0f6).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66169      +/-   ##
==========================================
- Coverage   92.75%   90.31%   -2.44%     
==========================================
  Files         421      790     +369     
  Lines      190606   272053   +81447     
  Branches    29206    51945   +22739     
==========================================
+ Hits       176790   245701   +68911     
- Misses      13488    16878    +3390     
- Partials      328     9474    +9146     

Files with missing lines	Coverage Δ
src/node_zlib.cc	79.95% <100.00%> (ø)

... and 501 files with indirect coverage changes

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