Stream help answer

What does this MR do and why?

Add feature flag to test streaming help message in Duo Chat.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable :duo_chat_stream_help_answer

2. In VS Code, ask /help and see the message

Related to #499479