Re-styled the AiGenieChat component to look like Tanuki Bot Chat

What does this MR do and why?

In Tanuki Bot Chat (!117597 - merged) we have introduced the Tanuki Bot Chat. In the course of Define and design the AI Chat Experience, this issue styles the AiGenieChat component to look like the Tanuki Bot as the first step of unification the chat interface in the product.

Screenshots or screen recordings

With explain_code_chat flag ON:

	Before	After
Little content
A lot of content

With explain_code_chat flag OFF:

	Before	After
Little content
A lot of content

How to set up and validate locally

1. Enable the following feature flags:
   • explain_code_snippet
   • explain_code_chat ⚠ this is the flag that works separately from all others and only controls the chat
   • openai_experimentation
   • ai_experimentation_api
2. Add an OpenAI API key by following the steps in !116364 (merged)
3. Open a file containing Source Code (Note, this file should not be from the GitLab codebase).
   • sample_file.js
4. Highlight some of the Source Code.
5. Click on the (?) button in the gutter to the left of the source code.

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.

References https://gitlab.com/gitlab-org/gitlab/-/issues/410106