Fix custom field object type check

[pitan]

Summary

Fixes a false-negative validation when applying custom_field_data from YAML initializers.
A custom field could be incorrectly reported as “not enabled for ”, even though it is enabled.

Root cause

ObjectType.objects.get_for_model() returns NetBox's ObjectType, while CustomField.object_types
is a queryset of Django ContentType (or a different type depending on NetBox version).
Because the membership check compares different model classes, ct not in cf.object_types.all()
can evaluate to True even when the underlying content type matches.

Changes

• Compare by primary key using .filter(pk=ct.pk).exists() instead of ct in queryset

Reproduction

1. Create a Custom Field assigned to IPAM > Prefix
2. Reference it under custom_field_data in prefixes.yml
3. Run load_initializer_data
   Before: prints “Custom field ... is not enabled for Prefix's model” and the value is not applied
   After: no false warning and the custom field value is applied

Tested

• Tested on: NetBox v4.4.10 / netbox-initializers v4.4.0