Object/multiobject required is enforced at the REST layer only, never at the model layer

[bctiemann]

Plugin Version

0.7.0

NetBox Version

4.7.1

Python Version

3.12

Steps to Reproduce

1. Create a Custom Object Type with an object (or multiobject) field marked "Required".
2. Create an instance directly via the ORM (e.g. model.objects.create(...)) leaving that field unset, and call full_clean() on it.
3. Compare against a scalar field type (e.g. text) marked "Required" undergoing the same full_clean() call.

Expected Behavior

full_clean() should reject the empty value for an object/multiobject field the same way it now does for every scalar field type (fixed in #700) — "Required" should mean the same thing regardless of field type.

Observed Behavior

full_clean() accepts the empty value. ObjectFieldType.get_model_field()/MultiObjectFieldType.get_model_field() hardcode blank=True on their generated model field(s) unconditionally — never tied to field.required — so required for these two types is enforced only at the REST serializer layer (already correct, pre-#700), never at the model layer.

Raised during review of #700/#714 (h/t @jnovinger): "Object and multiobject fields still hardcode blank=True... required on those two types stays REST-only. The scalar types now enforce at both layers, which leaves the same 'Required' checkbox meaning two different things depending on field type."

Suspected Cause

Same root cause shape as #700, deliberately out of scope there (#700's own reproduction excluded these two types, since their serializer-level required handling already worked correctly before #700 — only the model-level blank was never addressed). field_types.py:932, :937 (plain object's two backing columns), and the plain multiobject field's CustomManyToManyField all hardcode blank=True regardless of field.required.

Proposed Fix

Set blank=not field.required on ObjectFieldType/MultiObjectFieldType's generated model field(s) (the non-polymorphic case's FK/M2M columns), mirroring the scalar-type fix in #700. The two polymorphic variants (GFK-backed object, PolymorphicM2MDescriptor-backed multiobject) have no real, directly-queryable backing field to attach blank to in the same way — and #700/#714's required-toggle pre-flight check (added to CustomObjectTypeField.clean()) already explicitly treats a GenericForeignKey entry and any field without a usable .blank attribute as unaffected by this, so scope this fix to the plain (non-polymorphic) case only, unless a parallel mechanism is designed for the polymorphic ones.