Add support for reasoning content to Microsoft.Extensions.AI integration

[smitranic]

Currently Microsoft.Extensions.AI integration doesn't support reasoning content.

This PR maps reasoning content to TextReasoningContent on MEAI side.

Sample usage:

TornadoApi api = new(endpointUrl, apiKey);

var chatClient = api.AsChatClient(modelName);
List<ChatMessage> messages = [
    new(ChatRole.User, "Why is sky blue?")
];

var response = await chatClient.GetResponseAsync(messages);

// Standard message.Text usage (unchanged):
Console.WriteLine(response.Messages.FirstOrDefault()?.Text);

// Grab the TextContent only (unchanged):
Console.WriteLine(response.Messages.FirstOrDefault()?.Contents.OfType<TextContent>().FirstOrDefault()?.Text);

// Grab the TextReasoningContent only (now supported):
Console.WriteLine(response.Messages.FirstOrDefault()?.Contents.OfType<TextReasoningContent>().FirstOrDefault()?.Text);

[lofcz]

thanks!