fix(resolve_exe): allow shutil.which() to find exe without suffix in Windows

[martclanor]

This PR aims to address an issue found in local testing in Windows with Python 3.13.1. The issue is seen in the failing test: test_mbase.py::test_run_model_exe_rel_path and is caused by a behavior change in shutil.which (in Windows) introduced in python/cpython#127035.

For more context, in test_run_model_exe_rel_path, the mf6 executable is copied to a relative path, ../bin/mf6 (without extension). Within the test, shutil.which is used to locate the executable path. However, with the new changes in shutil.which, by default, it would only search for the path with an extension in PATHEXT, excluding paths without extension.

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.3%. Comparing base (bb9824e) to head (c3b2208).
Report is 26 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mbase.py	93.7%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2408     +/-   ##
=========================================
+ Coverage     68.4%   76.3%   +7.9%     
=========================================
  Files          294     294             
  Lines        59390   59781    +391     
=========================================
+ Hits         40652   45655   +5003     
+ Misses       18738   14126   -4612     

Files with missing lines	Coverage Δ
flopy/mbase.py	72.3% <93.7%> (+2.4%)	⬆️

... and 245 files with indirect coverage changes

[wpbonelli]

@martclanor there was an issue with the latest executables release, which is what gets installed for all these CI jobs. I deleted the release while we figure it out. If you restart I think the CI should pass now.