Use Duo icon and language for AI actions in editor

What does this MR do and why?

Replaces tanuki with tanuki-ai to align with other AI features and downstream UI from the AI actions menu (e.g. description generation prompt window uses tanuki-ai. This will help reinforce that this relates to AI features.

Additionally this aligns to "GitLab Duo" messaging by:

• Changing the button tooltip from "AI actions" to "GitLab Duo"
• Removing the redundant label in the dropdown

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Ensure AI features are accessible: https://docs.gitlab.com/ee/development/ai_features/index.html#saas-only-features
2. Create a new issue, or edit an MR, using Markdown editor