Messages do not serialize properly

[qandrew]

In the context of vLLM usage, when we try to serialize messages for sending it out as a SSE event, the serialization doesn't work. We wrote a workaround in vllm-project/vllm#26185 but it would be nice to fix it in this library.

parser.messages[0]
Message(author=Author(role=<Role.ASSISTANT: 'assistant'>, name=None), content=[TextContent(text='We need to respond as ChatGPT. The user says "Hello." We respond politely. Possibly ask how can help.')], channel='analysis', recipient=None, content_type=None)
parser.messages[0].model_dump_json()
'{"author":{"role":"assistant","name":null},"content":[{}],"channel":"analysis","recipient":null,"content_type":null}'

[bmillwood]

FWIW on the deserialization side we also see model_validate_json expects a different format from to_json

>>> import openai_harmony as oh
>>> oh.Message.model_validate_json(oh.Message.from_role_and_content(role=oh.Role.USER, content="hello").to_json())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/nix/store/3c6dkmzpx9davsx88yx6fa7ybfliswrc-python3-3.12.11-env/lib/python3.12/site-packages/pydantic/main.py", line 744, in model_validate_json
    return cls.__pydantic_validator__.validate_json(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.ValidationError: 1 validation error for Message
author
  Field required [type=missing, input_value={'role': 'user', 'name': ...ext', 'text': 'hello'}]}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/missing
```