Use tanuki-ai icon for AI features

What does this MR do and why?

Use tanuki-ai icon for AI features

Open questions

• Do we want to continue using it in each drawer header?
• Should it be purple? Orange? Or maybe gray-600 to match other icon + text instances?

Screenshots or screen recordings

Before	After

Other instances

Help menu	Vulnerability report > SAST vulnerability > explain this vulnerability	Source code > highlight line of code > click ? in gutter

MR diff > ellipsis icon in top right corner of file > Generate test	Issue > activity > summarize comments	Issue > edit > Autocomplete

How to set up and validate locally

Feature flags:

 :openai_experimentation
 :tanuki_bot
 :explain_code_chat
 :explain_code_snippet
 :ai_experimentation_api
 :ai_related_settings
 :tofa_experimentation_main_flag
 :anthropic_experimentation
 :explain_code
 :generate_test_file_flag
 :search_issue_label_aggregation
 :summarize_notes
 :summarize_comments
 :generate_description_ai
 :explain_vulnerability

#411677 (closed)