IDEX l1b Trigger origin, level, and mode fix

[lacoak21]

Change Summary

closes #2658

Overview

Fix trigger mode, level, and origin at l1b. Update the rate calculation to use the correct delay.

File changes

Replace merged single trigger_mode/trigger_level outputs with per-gain-channel trigger mode/level variables in L1B and add a new trigger_origin variable.
Update L1A sample trigger-time calculation to pick the delay based on the trigger ID bits, and fix time-index generation (linspace → arange).
Update tests and external validation fixtures/mappings to align with new variable names and updated validation-file contents.

Testing

The validation file from Ethan contains different formatting and variables than we expect. The l1b validation code handles this but we should get a new one eventually.

[Copilot]

Pull request overview

Fixes IDEX L1B trigger metadata handling to avoid merge conflicts and to better represent trigger configuration/origin per event, and updates L1A timing calculations to use the trigger-specific sample delay.

Changes:

• Replace merged single trigger_mode/trigger_level outputs with per-gain-channel trigger mode/level variables in L1B and add a new trigger_origin variable.
• Update L1A sample trigger-time calculation to pick the delay based on the trigger ID bits, and fix time-index generation (linspace → arange).
• Update tests and external validation fixtures/mappings to align with new variable names and updated validation-file contents.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
imap_processing/idex/idex_l1b.py	Emits per-channel trigger mode/level variables, adds trigger-origin extraction, and wires these into the L1B dataset.
imap_processing/idex/idex_l1a.py	Uses trigger ID to select the appropriate delay field when computing high-sample trigger time; updates time index construction.
imap_processing/cdf/config/imap_idex_l1b_variable_attrs.yaml	Replaces single trigger attrs with per-channel attrs and adds attrs for trigger_origin.
imap_processing/tests/idex/test_idex_l1b.py	Updates trigger-setting expectations and validation mappings; adjusts validation comparison logic for updated example file.
imap_processing/tests/idex/conftest.py	Switches the L1B example validation file name/path used by tests.
imap_processing/tests/external_test_data_config.py	Updates external test data download configuration for the renamed IDEX L1B validation file.

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[tech3371]

I looked at the code and code changes makes sense on high level. Thank you for testing though. It helps!