Sentry Issue: [POSTHOG-EU-3K](https://sentry.io/organizations/posthog/issues/3662211615/?referrer=github_integration)

```
ValidationError: {'events': [{'math': [ErrorDetail(string='"avg" is not a valid choice.', code='invalid_choice')]}]}
  File "posthog/api/insight.py", line 604, in trend
    serializer.is_valid(raise_exception=True)
  File "rest_framework/serializers.py", line 228, in is_valid
    raise ValidationError(self.errors)
```
posthog/models/entity/entity.py
return {
            "id": self.id,
            "type": self.type,
            "order": self.order,
            "name": self.name,
            "custom_name": self.custom_name,
            "math": self.math,
            "math_property": self.math_property,
            "math_group_type_index": self.math_group_type_index,
            "properties": self.property_groups.to_dict(),
        }
    def equals(self, other) -> bool:
        """Checks if two entities are semantically equal."""
        
        if self.id != other.id:
            return False
        if self.type != other.type:
            return False
        
        
        self_properties = sorted(str(prop) for prop in self.property_groups.flat)
        other_properties = sorted(str(prop) for prop in other.property_groups.flat)
        if self_properties != other_properties:
            return False
        return True
    def is_superset(self, other) -> bool: