Propagate model field defaults to ModelSerializer fields

[M2004GV]

Description

Refreshed resubmission of #8130, which was closed without being merged.

ModelSerializer already propagates a model field's default to the generated serializer field when that field participates in a uniqueness constraint (get_uniqueness_extra_kwargs), but not otherwise. This means a plain IntegerField(default=0) (with no uniqueness constraint) is generated as required=False but with no default, so omitting it from the input simply drops the attribute instead of falling back to the model's default — inconsistent with how Django's ModelForm behaves, and surprising given DRF already does the equivalent for unique-constrained fields.

This PR generalizes that existing behavior: whenever model_field.has_default(), the value is passed through as the serializer field's default in get_field_kwargs.

Closes #7469. Related to #2683 / #7489.

Addressing prior review feedback

On the original PR, @auvipy asked for two things before it could be accepted:

1. Rebase with a fresh approach — done here as a clean two-commit PR.
2. Update OPTIONS responses to include the default, consistent with how default is already surfaced in schemas/openapi.py. Done in the second commit: SimpleMetadata.get_field_info now includes a default key for any non-callable field default.

Behavior change

The only user-visible behavior change is for non-partial updates/creates: a field with a model-level default that is omitted from the input will now have that default appear in validated_data, matching what already happens for fields covered by uniqueness constraints. Partial updates (partial=True) are unaffected — defaults are still skipped in that case, per existing Field.get_default() behavior.

Test plan

• Added test_default_value_used_when_field_omitted and test_default_value_skipped_on_partial_update in tests/test_model_serializer.py, and updated the repr() regexes in test_regular_fields / test_field_options to reflect the new default=... kwarg.
• Added test_default_value_field_info and test_callable_default_field_info in tests/test_metadata.py.
• Added a note to docs/api-guide/fields.md documenting the propagation.
• Full test suite passes against both Django 5.2 and Django main (6.2 dev): 1562 passed, 0 failed.
• flake8 clean on changed files.