Summary

Preserve integer placeholder typing in the monthly IDEX count products.

This PR is a small part of the broader ISTP / metadata cleanup tracked in issue #2577. In that larger effort, this branch addresses one specific remaining file-level metadata mismatch in the monthly IDEX outputs by making the written count-variable types agree with their declared CDF metadata.

Scope:

• imap_idex_l2b_sci-1mo
• imap_idex_l2c_rectangular-map-1mo

Why

The monthly IDEX products intentionally blank several derived outputs while the science team continues validating the fitting routines and derived values.

Before this change, the placeholder block applied np.nan to both:

• floating-point rate variables
• integer count variables

That promoted the monthly count variables to floating-point in the written CDFs, while their metadata remained integer-style. The result was an ISTP/SPDF mismatch in the emitted files for:

• counts_by_charge
• counts_by_mass
• counts_by_charge_map
• counts_by_mass_map

So within the broader metadata cleanup under #2577, this PR fixes the IDEX monthly count-placeholder type mismatch at the source.

What Changed

Preserve integer placeholder typing for monthly counts

For the monthly count variables only, replace the NaN placeholder behavior with the standard int64 CDF fill sentinel:

• counts_by_charge
• counts_by_mass
• counts_by_charge_map
• counts_by_mass_map

These variables now:

• stay np.int64
• use -9223372036854775808 as the placeholder fill value

Leave monthly rate placeholders unchanged

The monthly rate placeholders are intentionally unchanged and still use NaN:

• rate_by_charge
• rate_by_mass
• rate_by_charge_map
• rate_by_mass_map

That remains valid because those variables are already float-typed and their metadata matches the written data.

Update tests accordingly

The monthly IDEX regression tests now:

• assert that the count placeholders are filled with the integer fill sentinel
• keep the existing NaN assertions for the rate placeholders
• verify that the written monthly count variables serialize as integer CDF types with the expected integer FILLVAL

Testing

Validated locally with:

pytest -q imap_processing/tests/idex/test_idex_l2b.py
pre-commit run --files \
  imap_processing/idex/idex_l2b.py \
  imap_processing/tests/idex/test_idex_l2b.py