ci(commit): add optional dependencies to the pixi environment

[jdhughes-dev]

The Test job runs in the MODFLOW 6 pixi environment and installs FloPy with --no-deps,
so none of the optional extras are present and every test guarded by requires_pkg() for
them is skipped on a pull request.

• add h5py, pymetis, and scikit-learn to the pixi environment rather than installing them
  from pypi, so they are consistent with the libraries already there
• h5py gates the node mapping save and load tests, pymetis and scikit-learn gate the metis
  splitting tests, none of which have run on a pull request
• conda-forge builds pymetis for windows, which pypi does not, so those tests run on all
  three platforms

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.6%. Comparing base (556c088) to head (0c72e9c).
⚠️ Report is 211 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2822      +/-   ##
===========================================
+ Coverage     55.5%    73.6%   +18.0%     
===========================================
  Files          644      659      +15     
  Lines       124135   132188    +8053     
===========================================
+ Hits         68947    97321   +28374     
+ Misses       55188    34867   -20321     

see 585 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[jdhughes-dev]

Windows failed on the first attempt: the h5py and netcdf4 wheels each bundle their own
HDF5 library and windows loaded the wrong one, so import h5py raised

ImportError: DLL load failed while importing _errors: The specified procedure could not be found.

requires_pkg() only catches ModuleNotFoundError, so this propagated out of has_pkg() and
the whole of test_model_splitter.py failed to collect rather than skipping. Pushed 2dd000b to
install h5py on linux and macOS only.

Worth noting separately: modflow_devtools.misc.has_pkg() catching only ModuleNotFoundError in
try_import() means a package that is installed but broken takes down collection instead of
reporting as unavailable. Catching ImportError there would make it degrade the way the strict
check intends.

[wpbonelli]

I think the windows failures are pypi/conda inconsistencies. the pixi env has hdf5 the library as a transitive conda dependency and then we install h5py from pypi.

Instead of tacking on pypi dependencies here we might as well just add these few to the mf6 pixi env, I think.

[jdhughes-dev]

Good call, that is the cause. Switched to pixi add h5py pymetis scikit-learn so they come from conda and match the HDF5 library already in the environment, and dropped the windows carve out. conda-forge also builds pymetis for windows where pypi does not, so the metis tests now run on all three platforms rather than being skipped.