refactor(netcdf): write input arrays only in validate mode

[mjreno]

Restrict writing gridded input arrays to NetCDF in validate mode only:

• documentation reflects package export_array_netcdf option is ignored if model output NetCDF is not configured or simulation is not run in validate mode
• NetCDF files generated in validate mode will have a time dimension of NPER, while those generated during a typical simulation run will have a time dimension of NSTP
• These changes align with FloPy4 development
• There is ongoing discussion related to the possibility of deprecating input NetCDF file generation from MODFLOW 6

Checklist of items for pull request

• Replaced section above with description of pull request
• Added new test or modified an existing test
• Ran ruff on new and modified python scripts in .doc, autotests, doc, distribution, pymake, and utils subdirectories.
• Formatted new and modified Fortran source files with fprettify
• Added doxygen comments to new and modified procedures
• Updated definition files
• Updated input and output guide
• Removed checklist items not relevant to this pull request

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