Minor UI chat improvements

What does this MR do and why?

Fixes spacing in the chat

1. Removes margin under the alert
2. Makes the question text in the chat centered
3. Removes white space above the footer(input)

Make propmpt_location dynamic, fix spacing in chat

Changelog: fixed

Screenshots or screen recordings

before	after

How to set up and validate locally

1. Enable the following feature flags:
   • explain_code
   • 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. Run the following command in the root of your project:
   • curl https://gitlab.com/gitlab-org/gitlab/-/merge_requests/117636.diff | git apply
   • This will apply changes from !117636 (merged) on which this MR relies.
4. Open a file containing Source Code (Note, this file should not be from the GitLab codebase).
   • sample_file.js
5. Highlight some of the Source Code.
6. 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.