Summarize notes frontend

What does this MR do and why?

Frontend changes extracted from Draft: Summarize notes draft (!116583 - closed)

Adds a new "AI actions" dropdown within the markdown editor on the issues page. This includes a single "Summarize comments" action.

The component subscribes to "AI response"s and populates the markdown editor textbox with completions when they are available.

These changes are hidden behind a new feature flag: summarize_comments and also require the existing feature flag openai_experimentation to be enabled.

Screenshots or screen recordings

before / FF disabled	new dropdown	loading	generic error handling

(note the screenshots have incorrect casing Summarize Comments which is now Summarize comments as in the gif)

How to set up and validate locally

• Enable feature flags

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

• Set an openai_api_key in your ApplicationSettings.

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

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.

Related to #406619