Add tooltip for AI actions button

What does this MR do and why?

Adds a tooltip to the AI actions button. No changelog because it's behind a feature flag.

Screenshots or screen recordings

How to set up and validate locally

• Enable feature flags

  Feature.enable(:openai_experimentation)
  Feature.enable(:summarize_comments)

• Set an openai_api_key in your ApplicationSettings.

• Visit an issue, and scroll down to write a comment. Note the new AI Actions dropdown in the markdown editor toolbar

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.