Add new max tokens field

[danai-antoniou]

adding MaxCompletionTokens to make our chat completions compatible with the o1 models, otherwise we fail with:

HTTP 500 Internal Server Error: { "code": "unknown", "message": "error, status code: 400, message: Unsupported parameter: 'max_tokens' is not supported with this model. Use 'max_completion_tokens' instead.", "details": null }

Also from the docs