feat(mfusgwel): add comment column support for MfUsgWel package

[reneangermeyer]

Groundwater Vistas (and other preprocessors) commonly export MODFLOW-USG WEL
files with undeclared trailing text after the numeric data — such as well names,
aquifer zone IDs, recharge source identifiers, or other user annotations. These
are not declared AUX variables and are not numeric.

This PR captures that trailing text as comment columns (object dtype) so they
are preserved on read/write roundtrip without being incorrectly declared as
AUX variables in the header.

Example

Input (GWV export, no AUX declared for trailing text):
52901 451.694 70.000 9999 i1_pul
1 -64.360 1150.199 1 PA-001

• 9999 / 1 → zone or aquifer ID (comment1)
• i1_pul / PA-001 → well name (comment2)

Changes

• flopy_io.py: ulstrd() captures extra tokens beyond declared columns
• pakbase.py: Package.load() expands dtype with comment columns
• mfusgwel.py: _check_for_aux() skips object dtype fields, get_empty()
  supports comment columns
• mfwel.py: parent class aux detection also skips object dtype fields

[codecov]

Codecov Report

❌ Patch coverage is 82.14286% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.3%. Comparing base (556c088) to head (0092ae7).
⚠️ Report is 117 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/pakbase.py	78.2%	15 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2695      +/-   ##
===========================================
+ Coverage     55.5%    72.3%   +16.8%     
===========================================
  Files          644      667      +23     
  Lines       124135   130552    +6417     
===========================================
+ Hits         68947    94512   +25565     
+ Misses       55188    36040   -19148     

Files with missing lines	Coverage Δ
flopy/mfusg/mfusgwel.py	96.2% <100.0%> (+4.4%)	⬆️
flopy/modflow/mfwel.py	87.7% <100.0%> (-2.3%)	⬇️
flopy/utils/flopy_io.py	72.3% <100.0%> (-3.1%)	⬇️
flopy/pakbase.py	82.1% <78.2%> (-2.4%)	⬇️

... and 559 files with indirect coverage changes

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[jdhughes-dev]

This needs a test to confirm read/write with comments. Also would need a test of adding comments to a fresh model instance.