`schema` must be present and not `null` when `text.format.type` is `json_schema`

[nascent-david]

This is used to enable structured model outputs.

schema in async_openai::types::assistants::ResponseFormatJsonSchema is an Option (which is skipped if None), but this contradicts OpenAI's official type from the Python SDK, where it is not Optional.

OpenAI's API is accommodating and will fall back to disabling structured outputs if you do not provide a schema, but other implementations, like VLLM, are not, and will reject your request:

{
'type': 'missing', 'loc': ('body', 'text', 'format', 'ResponseFormatTextJSONSchemaConfig', 'schema'),
   'msg': 'Field required',
...

[64bit]

This is from the OpenAPI spec, where its not a required field for ResponseFormatJsonSchema.

However, it does make sense semantically that one should always provide schema - PR is welcome