feat(mfusgwel): add boundnames support for well package

[reneangermeyer]

Add support for reading and writing well names (boundnames) in the MODFLOW-USG WEL package. Since MODFLOW-USG doesn't natively support the BOUNDNAMES keyword, boundnames are implemented as auxiliary variables.

Changes:

• Add boundnames parameter to MfUsgWel.init for creating wells with boundnames
• Add string_aux_names parameter to MfUsgWel.get_empty() for loading files with string-type auxiliary variables
• Auto-detect extra columns (including boundnames) when loading WEL files that don't declare them in the header (common with Groundwater Vistas exports)
• Infer dtype from loaded data to handle auto-detected columns
• Update ulstrd() to handle object-type columns and expand dtype when extra columns are detected

The implementation supports:

• Creating new wells with boundnames using boundnames=True
• Custom dtype with boundname field for both GWF and CLN wells
• Loading existing files with boundnames (auto-detected)
• Writing files with proper aux boundname declarations

[codecov]

Codecov Report

❌ Patch coverage is 59.52381% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.3%. Comparing base (556c088) to head (1d15b33).
⚠️ Report is 115 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/pakbase.py	56.5%	30 Missing ⚠️
flopy/mfusg/mfusgwel.py	57.1%	3 Missing ⚠️
flopy/modflow/mfwel.py	50.0%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2693      +/-   ##
===========================================
+ Coverage     55.5%    72.3%   +16.8%     
===========================================
  Files          644      667      +23     
  Lines       124135   130546    +6411     
===========================================
+ Hits         68947    94464   +25517     
+ Misses       55188    36082   -19106     

Files with missing lines	Coverage Δ
flopy/utils/flopy_io.py	72.3% <100.0%> (-3.1%)	⬇️
flopy/modflow/mfwel.py	86.8% <50.0%> (-3.2%)	⬇️
flopy/mfusg/mfusgwel.py	90.1% <57.1%> (-1.8%)	⬇️
flopy/pakbase.py	80.0% <56.5%> (-4.4%)	⬇️

... and 559 files with indirect coverage changes

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