Missing ChatGPT5 chat completions fields

[the-spice-must-flow]

There are fields and options missing from ChatCompletionsRequestParams that became available with the launch of GPT-5

Created an issue in the OpenAPISpec as it feels reasonable this package only supports what's officially published. openai/openai-openapi#481

But just in case the team wants to tackle these sooner it would be much appreciated:

Minimal variant for reasoning effort ✅ (already done #411 -- but missing from spec)
verbosity field
allowed-tools options

Since there are breaking changes in the api depending on the model you specify is this possible to reflect in the crate? Either with enums or feature flags?

That may be out of scope but just some thoughts on this.

I would submit a PR if it was as easy as just amending the ChatCompletionsRequest for these missing fields.

[joune]

#439