Implement mergeRequestDiffs property for MergeRequestType

What does this MR do and why?

This adds mergeRequestDiffs property to MergeRequestType in GraphQL API.

It returns the following properties:

• diff_llm_summary - returns a MergeRequestDiffLlmSummaryType
• review_llm_summaries - returns a collection of MergeRequestReviewLlmSummaryType. Supports cursor pagination.
• created_at
• updated_at

How to set up and validate locally

1. Run migration: rake db:migrate.

2. Create a MR.

3. Submit a review on the MR.

4. Create MergeRequest::ReviewLlmSummary record for the submitted review via rails console:

   MergeRequest::ReviewLlmSummary.create(merge_request_diff: MergeRequestDiff.last, review: Review.last, user: User.llm_bot, provider: :open_ai, content: "Review summary")

5. Make this GraphQL query:

   query {
     project(fullPath: "<insert path of project>") {
       id
       mergeRequest(iid: "<insert IID of MR>") {
         id
         mergeRequestDiffs {
           edges {
             cursor
             node {
               diffLlmSummary {
                 user {
                   id
                 }
                 provider
                 content
                 createdAt
                 updatedAt
               }
               reviewLlmSummaries {
                 edges {
                   cursor
                   node {
                     user {
                       id
                     }
                     reviewer {
                       id
                     }
                     provider
                     content
                     createdAt
                     updatedAt
                   }
                 }
               }
             }
           }
         }
       }
     }
   }

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 #415359 (closed)