Replace experimentation API with GraphQL endpoint for Explain Code

What does this MR do and why?

This replaces the current experimentation API for Explain Code with the Abstraction layer (GraphQL endpoint for AI Explain Code) on the frontend.

• Related Backend MR

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. 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/117358.diff | git apply
   • This will apply changes from !117358 (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.
7. A popup should appear and the response should load once we receive a notification from the backend.

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 #406783