Fill in merge request template using code changes

This is an extension to Detailed description of proposed changes in a m... (#409375).

Idea Details

When a merge request is proposed, there may be lots of changes and details about those changes. As part of &10223 we're providing a 2-3 sentence summary of the changes, but for the purposes of the description of a merge request, it would be better to have a more detailed description that covers all of the lines and changes.

Proposal

We should add a quick action to the Create new merge request editor that could fill in the existing template using the code changes from the selected branch. It should be as detailed as possible about the changes and exclude sections it's uncertain about (for example reproduction steps).

Availability and Testing

Add spec to ensure the correct prompt is being sent to generate a detailed summary. Add feature spec to ensure that quick action is functional using a stubbed AI endpoint.