Consider Adding ChatResponse.ReasoningText

[rogerbarreto]

With the growth of reasoning models and the presence of TextReasoningContent would be desirable to have a easier way to extract all the thinking content from ChatResponse.

ChatResponse response = chatClient.GetResponseAsync(...);
string? text = response.Text;
string? thinking = response.ReasoningText; // GG

This would avoid the complexity of combining multiple message thinking together like below:

    string assistantThinking = string.Join("\n", result.Messages
        .SelectMany(m => m.Contents)
        .OfType<TextReasoningContent>()
        .Select(trc => trc.Text));

[stephentoub]

I'm not sure how desirable this actually is. Anything like this we add increases concept count on the surface area of these types, and the operation it would perform is the same as a user can do on their own, e.g. using an approach like that cited with LINQ. There's also other forms of reasoning content that couldn't be represented, e.g. encrypted reasoning content from Anthropic or OpenAI. And there are other kinds of content where developers want to extract all of the AIContent-instances of a particular type from across a whole response; I don't think we should be creating accelerators for each of them. I'd rather "wait and see" before adding more here.