external_id_mapping fails for non compound unique indexes with underscores

[lsmith77]

The RestApiHandler's external_id_mapping configuration incorrectly parses single-field unique indexes that contain underscores in their field names, treating them as compound IDs instead.

Current Behavior:

When using external_id_mapping with a single field name containing underscores:

external_id_mapping: {
  MyModel: 'external_unique_id'
}

Error:

The handler splits 'external_unique_id' by underscores into ['external', 'unique', 'id'] and attempts to find fields with those names, failing when they don't exist as separate fields.

Expected Behavior
external_id_mapping should support:

• Single-field unique indexes: { MyModel: 'external_unique_id' } where external_unique_id is one field
• Compound unique indexes: { MyModel: 'field1_field2' } where field1 and field2 are separate fields

Fixes

I see two possible fixes:

1. Check if a field exists before splitting
2. Optionally allow passing in an array of field names

For 1) it is a bit iffy, since this only really fixes the issue for non-compound columns. But since in theory the issue could also affect compound indexes, 2) would be cleaner.

[lsmith77]

this bug also affects v3 zenstackhq/zenstack-v3#611