Add ai_response_time tracking event definition

What does this MR do and why?

This commit adds the missing ai_response_time tracking event definition.

Which was missing on this MR: !153221 (merged)

Screenshots or screen recordings

No visual changes.

How to set up and validate locally

1. Setup Duo Chat: https://docs.gitlab.com/ee/development/ai_features/duo_chat.html#set-up-gitlab-duo-chat
2. Open up Duo Chat
3. Ask question
4. Install Snowplow Analytics Debugger Extension to visualize the tracking events