Add "ExtraHigh" value to `ResponseReasoningEffortLevel` extensible enum

[joseharriaga]

"xhigh" in the REST API.

For more info, see: https://developers.openai.com/api/reference/resources/responses/methods/create#(resource)%20responses%20%3E%20(method)%20create%20%3E%20(params)%200.non_streaming%20%3E%20(param)%20reasoning%20%3E%20(schema)%20%2B%20(resource)%20%24shared%20%3E%20(model)%20reasoning%20%3E%20(schema)%20%3E%20(property)%20effort

To validate, test that the returned ResponseResult echoes back this value in its ReasoningOptions.ReasoningEffortLevel property.