Add GraphQL endpoint for review summary

Background

We need to provide information via GraphQL about the review summaries generated by AI.

Proposal

1. Introduce merge_request_review_llm_summary with the following columns:

   • id
   • merge_request_diff_id (this will be the latest merge request diff at the time the review summary is created)
   • review_id (the associated Review record)
   • user_id (user that creates the summary, will be User.llm_bot just like for diff summary)
   • provider
   • content
   • created_at
   • updated_at

2. Introduce mergeRequestDiffs field. See #415359 (comment 1439224762) for more details.

Availability and Testing

Add spec/graphql tests for new GraphQL endpoint