Add tanuki-ai icon to dropdown items and buttons

What does this MR do and why?

Add tanuki-ai icon to dropdown items and buttons

Questions

• Should we keep the current GitLab Chat dropdown item icon purple?
  • I made the Suggest test cases icon purple too for now
• Should the icon be aligned another way in the dropdown? 🤔

Screenshots or screen recordings

Before	After

How to set up and validate locally

• AI set up
• Look at following features:
  • Issue > View summary
  • Issue > edit > Autocomplete
  • Suggest test cases
  • Explain vulnerability
  • Help > GitLab Chat (no change)

#411677 (closed)

#414761 (closed)