Fix Hi L2 naming of bg variables

[subagonsouth]

Change Summary

Overview

This just renames the Hi L2 variables:

• bg_rates -> bg_rate
• bg_rates_unc -> bg_rate_sys_err

Closes: #2676

[Copilot]

Pull request overview

This PR aligns IMAP-Hi L2 background variable names with the established ENA maps L2 naming convention (per Issue #2676), renaming the background rate fields to their singular + systematic-error forms.

Changes:

• Renamed Hi L2 background variables: bg_rates → bg_rate, bg_rates_unc → bg_rate_sys_err.
• Updated Hi L2 processing code to reference the new background variable names.
• Updated unit tests and HiPointingSet expectations to match the renamed variables.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
imap_processing/hi/hi_l2.py	Updates projected/processed variable sets and computations to use bg_rate / bg_rate_sys_err.
imap_processing/ena_maps/ena_maps.py	Updates Hi L1C→L2 variable rename mapping to output bg_rate / bg_rate_sys_err.
imap_processing/tests/hi/test_hi_l2.py	Renames test datasets/assertions to use the new background variable names.
imap_processing/tests/ena_maps/test_ena_maps.py	Updates HiPointingSet test expectations to the new variable names.

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[Copilot]

ena_intensity_sys_err is currently computed from sqrt(bg_rate * exposure_factor) / exposure_factor, which is a Poisson/statistical uncertainty estimate based on background counts. This (a) doesn’t use the newly carried bg_rate_sys_err variable at all, and (b) conflicts with the CDF metadata describing ena_intensity_sys_err as a non-statistical/systematic error. Consider propagating bg_rate_sys_err into intensity units (and/or combining it with other systematic terms), or rename/reclassify this output so statistical vs systematic uncertainties are not mislabeled.