Align GitLab chat drawer headers

What does this MR do and why?

Align GitLab chat drawer headers

• Use neutral badge variant
• Use default badge size
• Use smaller text size for title
• Add tanuki icon on test case drawer

Screenshots or screen recordings

	Before	After
GitLab chat
Explain this source code
Suggest test cases

How to set up and validate locally

• Follow instructions in https://docs.gitlab.com/ee/development/ai_features.html#test-ai-features-locally
• Check GitLab chat (from help dropdown)
• Check Explain this code on any source code
• Check Suggest test cases from any MR diff file ellipsis dropdown

#411677 (closed)