Add ChoiceField support to FilterModifierMixin

Enable filter modifiers for single-choice ChoiceFields in addition to the
existing MultipleChoiceField support. ChoiceFields can now display modifier
dropdowns with "Is", "Is Not", "Is Empty", and "Is Not Empty" options when
the corresponding FilterSet defines those lookups.

The mixin correctly verifies lookup availability against the FilterSet, so
modifiers only appear when multiple lookup options are actually supported.
Currently most FilterSets only define 'exact' for single-choice fields, but
this change enables future FilterSet enhancements to expose additional
lookups for ChoiceFields.

netbox/utilities/forms/mixins.py

@@ -151,6 +151,12 @@ class FilterModifierMixin:
             (MODIFIER_EMPTY_TRUE, _('Is Empty')),
             (MODIFIER_EMPTY_FALSE, _('Is Not Empty')),
         ],
+        forms.ChoiceField: [
+            ('exact', _('Is')),
+            ('n', _('Is Not')),
+            (MODIFIER_EMPTY_TRUE, _('Is Empty')),
+            (MODIFIER_EMPTY_FALSE, _('Is Not Empty')),
+        ],
         forms.MultipleChoiceField: [
             ('exact', _('Is')),
             ('n', _('Is Not')),