Use TextReasoningContent in OpenAIResponseClient's IChatClient

[stephentoub]

The responses API supports reasoning content. We should be translating it to TextReasoningContent, for both non-streaming and streaming.

(I've not done so yet because thus far I've not been successful in getting OpenAI or Azure OpenAI to generate any reasoning summary content, with any model.)

[stephentoub]

This is blocked on waiting for the next release of the OpenAI library.