fix(validators): guard UniqueConstraint validators against queryset instance in ListSerializer (fixes #9484)

[AdarshJ173]

Problem

Fixes #9484 — AttributeError: 'BaseQuerySet' object has no attribute 'pk' when using ModelSerializer with many=True on a model that has a UniqueConstraint.

Root Cause

When many=True is passed to a ModelSerializer, many_init creates a ListSerializer wrapping the child serializer. The instance kwarg (a QuerySet) is forwarded to both the ListSerializer and the child serializer. When the child's UniqueTogetherValidator (or UniqueValidator) runs during validation, it calls:

def exclude_current_instance(self, attrs, queryset, instance):
    if instance is not None:
        return queryset.exclude(pk=instance.pk)  # 💥 QuerySet has no .pk
    return queryset

Because instance is the entire QuerySet, not a model instance, .pk raises AttributeError.

Reproducer (from #9484)

class Pet(models.Model):
    name = models.CharField(max_length=100)
    animal_type = models.CharField(max_length=100)
    can_fly = models.BooleanField(null=True)

    class Meta:
        constraints = [
            UniqueConstraint(fields=["name", "animal_type"], name="unique_pet")
        ]

class PetSerializer(serializers.ModelSerializer):
    class Meta:
        model = Pet
        fields = ('name', 'animal_type', 'can_fly')

class PetListSerializer(serializers.ListSerializer):
    child = PetSerializer()

    def create(self, validated_data):
        return Pet.objects.bulk_create([Pet(**item) for item in validated_data])

new_data = [
    {"name": "Polly", "animal_type": "Parrot", "can_fly": True},
    {"name": "Penguin", "animal_type": "Bird", "can_fly": False},
]
serializer = PetListSerializer(Pet.objects.all(), data=new_data, many=True)
serializer.is_valid(raise_exception=True)  # AttributeError: 'BaseQuerySet' object has no attribute 'pk'

Fix

Introduces a small private helper _is_single_instance(instance) that returns True only if instance is a non-None, non-QuerySet, non-list object that has a .pk attribute. Both UniqueValidator.exclude_current_instance and UniqueTogetherValidator.exclude_current_instance (and BaseUniqueForValidator.exclude_current_instance) now use this helper instead of if instance is not None.

This is minimal and fully backward-compatible:

• Single-instance update: unchanged — instance.pk still used to exclude the current object.
• Bulk create (ListSerializer + queryset instance): validators no longer crash; treat each child item as a create.
• None instance (plain create): unchanged.

The fix also corrects UniqueTogetherValidator.filter_queryset and __call__ which had the same latent bug of reading from serializer.instance without guarding against a queryset type.

Checklist

• Minimal, targeted fix (one helper, three call sites updated)
• Backward compatible — no API changes
• Fixes crash in UniqueValidator, UniqueTogetherValidator, and BaseUniqueForValidator
• Covers the filter_queryset path in UniqueTogetherValidator (latent bug: getattr(serializer.instance, source) would also crash if instance is a queryset)

[auvipy]

this will also need appropriate test coverage to verify