Add source_url to gitlab chat graphql response

What does this MR do and why?

Related: https://gitlab.com/gitlab-org/enablement-section/tanuki-bot/-/issues/31

Populate the source_url to the graphQL version of GitLab chat backend. This is currently done in the Rest API version here.

This is needed for the frontend to correct display links.

Screenshots or screen recordings

How to set up and validate locally

1. enable gdk for pgvector: gdk config set pgvector.enabled true
2. enable the embedding database: gdk config set gitlab.rails.databases.embedding.enabled true
3. gdk reconfigure
4. Enable checks: Feature.enable(:openai_experimentation), Feature.enable(:tanuki_bot)
5. Add openAI api key: Gitlab::CurrentSettings.update(openai_api_key: "<openai-api-token>")
6. Enable super sidebar: Feature.enable(:super_sidebar_nav)
7. Enable the new nav in the GitLab UI for gdk
8. Navigate to Help and select Ask the GitLab Chat

9. Ask a question
10. Verify the links are working (they should link to gitlab.docs.com)

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.