Update vulnerability drawer to match others

What does this MR do and why?

Update vulnerability drawer to match others (follows same pattern as Explain this source code, Suggest test cases, and GitLab chat)

• Increase z-index so the header isn't hidden
• Add tanuki and badge
• Move drawer subtitle to content

What does this MR not do?

• Empty state
• Loading state

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the openai_experimentation, ai_experimentation_api, and explain_vulnerability feature flags.
2. Clone this project: https://gitlab.com/gitlab-examples/security/security-reports (or use any project with a SAST vulnerability)
3. Run a pipeline against the master branch.
4. Go to Security -> Vulnerability report.
5. Click on a SAST vulnerability to go to the details page.
6. Click on the "Explain this vulnerability" button.

#411677 (closed)