Serializer for extra fields not used

[Viicos]

From https://github.com/pydantic/pydantic-core/pull/1835/files#r2425738743:

from pydantic import BaseModel

class Model(BaseModel, extra='allow'):
    a: int
    __pydantic_extra__: dict[str, int]


m = Model(a=1, extra=1)
m.a = 'not_an_int'
m.model_dump_json()  # PydanticSerializationUnexpectedValue warning raised

m.a = 1
m.extra = 'not_an_int'
m.model_dump_json()  # No warning raised

Also https://github.com/pydantic/pydantic-core/pull/1835/files#r2425737760 should probably be supported.