[LindseyZ1205]

Description

refs #9257

ModelSerializer makes a field optional when its model field has a default, but not when it has a db_default (new in Django 5.0), so the field is required even though the database fills it in. For example, with

from django.db import models
from django.db.models.functions import Now


class Article(models.Model):
    title = models.CharField(max_length=100)
    status = models.CharField(max_length=10, db_default="draft")
    created = models.DateTimeField(db_default=Now())

a ModelSerializer with fields = "__all__" rejects {"title": "Hello"} with "This field is required." for status and created. The workaround mentioned in #9257 is to set required=False in extra_kwargs for each such field.

This change also checks has_db_default() where get_field_kwargs() and get_relation_kwargs() set required=False for a field with a default, and adds db_default to the required docs. Field.has_db_default() exists since Django 5.2, the oldest version DRF supports. A field with a db_default that is part of a unique constraint stays required, as before, since UniqueTogetherValidator needs its value.

Tests: test_db_default_field_not_required (a constant and an expression db_default) and test_foreign_key_with_db_default in tests/test_model_serializer.py fail without this change. The test suite passes with Django 6.1.2 and Django main (1656 passed, 13 skipped, on Python 3.14). With the model above, the serializer now saves {"title": "Hello"}, and the database sets status to "draft" and created to the current time.