Fix ESMF file loading by forcing netCDF4 engine for .nc files

Resolves scipy netCDF reader IndexError when reading ESMF files.
The scipy reader fails on certain netCDF files with 'index 0 is out
of bounds' error. Force netCDF4 engine for .nc files to ensure
robust file reading across all platforms.

Fixes:
- test/io/test_esmf.py::test_read_esmf
- test/io/test_io_common.py::TestIOCommon::test_return_type[grid_from_format4]

Author: rajeeja

uxarray/core/api.py

@@ -107,6 +107,9 @@ def open_grid(
 
     else:
         # Attempt to use Xarray directly for remaining input types
+        # Force netCDF4 engine for .nc files to avoid scipy reader issues
+        if isinstance(grid_filename_or_obj, (str, os.PathLike)) and str(grid_filename_or_obj).endswith('.nc'):
+            kwargs.setdefault('engine', 'netcdf4')
         grid_ds = xr.open_dataset(grid_filename_or_obj, chunks=grid_chunks, **kwargs)
         grid = Grid.from_dataset(grid_ds, use_dual=use_dual)