Schema partial=True ignored for location="json"

I am attempting to have an option JSON body, but the user of `partial` does not work when `location="json"`

```python
bp = Blueprint(...)

@bp.route("")
class ModelsResource(MethodView):
@bp.response(HTTPStatus.OK, ModelSchema)
@bp.arguments(
ModelSchema(exclude=("model_id", "parameter_schema"), partial=True), # Partial works
location="query",
as_kwargs=True,
)
@bp.arguments(
ModelSchema(only=("parameter_schema",), partial=True), # Partial does *not* work
location="json",
as_kwargs=True,
)
def put(self, **kwargs):
result = db.session.scalar(
sqlalchemy.insert(Model).values(**kwargs).returning(Model)
)
db.session.commit()
return result
```

Am I doing something incorrect in the above example?

Start with the @bp.arguments decorators in the provided example, comparing the query location where partial=True works with the JSON location where it does not. Reproduce the behavior using ModelSchema(only=("parameter_schema",), partial=True) and check the JSON argument-loading path. Done means partial=True behaves consistently for JSON, or the limitation is clearly documented.

Written by the indexing model from the issue text.