fix(evt): fix optional field handling at write time

[wpbonelli]

EVT packages with nseg > 1, and auxiliary variables, failed on file write with:

  IndexError: invalid index (7)
  MFDataException: An error occurred while processing auxiliary variables in the "_get_file_entry_record" method.

When nseg > 1, the EVT package has optional fields pxdp, petm, petm0 that may or may not be present in the structured array depending on nseg and surf_rate_specified values. #2496 added _dependent_opt() and _optional_nval() methods to handle these fields when reading, but _get_file_entry_record() in mfdatalist.py still iterated through all DFN-defined fields when writing. The index then advanced past the array bounds before reaching auxiliary variables.

At write time, skip EVT-specific optional fields that aren't present in the structured array, using the same methods #2496 added for reading.

Close #2683

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.3%. Comparing base (556c088) to head (e12b6db).
⚠️ Report is 110 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2684      +/-   ##
===========================================
+ Coverage     55.5%    72.3%   +16.7%     
===========================================
  Files          644      667      +23     
  Lines       124135   130124    +5989     
===========================================
+ Hits         68947    94134   +25187     
+ Misses       55188    35990   -19198     

Files with missing lines	Coverage Δ
flopy/mf6/data/mfdatalist.py	64.1% <100.0%> (-5.3%)	⬇️

... and 562 files with indirect coverage changes

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