fix(api): Fix schema and field definitions for OpenAPI

Add `get_internal_type()` to custom field classes for Django compatibility,
annotate path parameters and operation IDs for background endpoints, and
provide serializer context on the RQ base viewset to clear schema warnings.

Fixes #20365

netbox/extras/api/serializers_/customfields.py

@@ -26,6 +26,7 @@ class CustomFieldChoiceSetSerializer(ChangeLogMessageSerializer, ValidatedModelS
             max_length=2
         )
     )
+    choices_count = serializers.IntegerField(read_only=True)
 
     class Meta:
         model = CustomFieldChoiceSet