[leowerneck]

Overview

Closes #348. Closes IMAP-Science-Operations-Center/imap_processing#3579.

Description

#344 stopped casting FILLVAL, VALIDMIN, and VALIDMAX to the variable's type for all time-typed variables (CDF_EPOCH, CDF_EPOCH16, CDF_TIME_TT2000). That was only needed for non-integer values that can't be represented as TT2000 and overflow on read-back. Integer attributes, the usual case for TT2000 variables, are now written as CDF_INT8 instead of CDF_TIME_TT2000, breaking ISTP compliance.

This PR skips the cast for time-typed variables only when the attribute value is not an integer. Integer values are cast to the variable's type as they were before #344, and non-integer values are still left alone, so the #343 fix is kept.

Two regression tests are added to tests/test_xarray_reader_writer.py:

• test_xarray_to_cdf_casts_istp_integer_epoch_attributes_to_tt2000: integer attributes on a TT2000 variable are written as CDF_TIME_TT2000, and integer attributes on a non-time variable stay CDF_INT8. Fails on main.
• test_xarray_to_cdf_preserves_istp_float_epoch_attributes: float attributes on a TT2000 variable are not cast to TT2000 and round-trip through cdf_to_xarray without overflowing. Fails if the cast is applied unconditionally, as it was before Fix datetime conversion failures observed with NumPy 2.5 #344.

Testing

Running:

uv run pytest --remote-data

results in 113 passed.