fix(mfusg): fix duplicate unit numbers in NAM file for MFUSG external files

[reneangermeyer]

When loading a model, DATA entries from the NAM file were being added to external_units even when they already existed in output_units (e.g., IUNITAFR from WEL package). This caused duplicate entries in the written NAM file.

Added check for output_units in _prepare_external_files() to prevent adding unit numbers that are already registered as output files.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.6%. Comparing base (556c088) to head (36fbbae).
⚠️ Report is 89 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2659      +/-   ##
===========================================
+ Coverage     55.5%    72.6%   +17.1%     
===========================================
  Files          644      667      +23     
  Lines       124135   129356    +5221     
===========================================
+ Hits         68947    93986   +25039     
+ Misses       55188    35370   -19818     

Files with missing lines	Coverage Δ
flopy/mfusg/mfusg.py	78.2% <100.0%> (+0.7%)	⬆️

... and 558 files with indirect coverage changes

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[wpbonelli]

would be ideal to have a minimal test for this but presumably it is confirmed to work on your end @reneangermeyer