def test_plain_field_validator_serialization() -> None:
    """`PlainValidator` internally creates a wrap ser. schema. This tests that we can
    still generate a JSON Schema in `'serialization'` mode.
    """

    class Foo(BaseModel):
        a: Annotated[int, PlainValidator(lambda x: x)]

    assert Foo.model_json_schema(mode='serialization') == {
        'properties': {'a': {'title': 'A', 'type': 'integer'}},
        'required': ['a'],
        'title': 'Foo',
        'type': 'object',
    }
`PlainValidator` internally creates a wrap ser. schema. This tests that we can
    still generate a JSON Schema in `'serialization'` mode.
    
tests/test_json_schema.py