Add AI actions dropdown to Epics

What does this MR do and why?

Follow up from Summarize notes frontend (!117327 - merged) Related to #408912 (closed)

Add the new AI Actions dropdown to Epics.

Also address reviewer suggestion to clear error message when clicking the button again, and ensure we log all error details when available.

Screenshots or screen recordings

epic page	work item (incident)

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.

Note: these flags are only exposed to the frontend on EE

Verify the AI actions dropdown appears in the markdown editor for epics, in addition to issues.

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.

Related to #406619