chore(mf6-codegen): defensive devtools compatibility fix

[wpbonelli]

Compatibility fix anticipating some minor changes in devtools. Comment header lines will soon be returned verbatim, without stripping the leading "# [flopy] [mf6]".

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.5%. Comparing base (556c088) to head (1508da4).
⚠️ Report is 165 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/utils/codegen/filters.py	0.0%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2762      +/-   ##
===========================================
+ Coverage     55.5%    72.5%   +17.0%     
===========================================
  Files          644      677      +33     
  Lines       124135   132680    +8545     
===========================================
+ Hits         68947    96260   +27313     
+ Misses       55188    36420   -18768     

Files with missing lines	Coverage Δ
flopy/mf6/utils/codegen/filters.py	0.0% <0.0%> (ø)

... and 579 files with indirect coverage changes

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