Add experiment label to ai chat header

What does this MR do and why?

Adds info badge to notify user that this is an experimental feature

Screenshots or screen recordings

How to set up and validate locally

1. Enable the following feature flags:
   • explain_code
   • explain_code_snippet
   • openai_experimentation
   • ai_experimentation_api
2. Open a file containing Source Code (Note, this file should not be from the GitLab codebase).
   • sample_file.js
3. Highlight some of the Source Code.
4. Click on the (?) button in the gutter to the left of the source code.
5. A popup should appear with the label on top.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #406562