Add tags field to Commit GraphQL type

What does this MR do and why?

Contributes to #588848 (closed)

Problem

The Vue-based commit list page needs to display tag badges per commit, but the Commit GraphQL type does not expose tag data. The existing HAML view gets tags via CommitPresenter#tags_for_display after a batch load_tags call, but this is not available to GraphQL consumers.

Solution

Add a tags field to CommitType that returns tag names pointing to a commit, delegating to CommitPresenter#tags_for_display. Include the LooksAhead concern in CommitsResolver to conditionally call CommitCollection#load_tags only when the tags field is in the GraphQL selection set. This makes a single Gitaly ListRefs RPC for all commits on the page, avoiding N+1 queries while skipping the call entirely when tags are not requested.

References

• Add multiple tags support to commit list GraphQ... (#588848 - closed)

Screenshots or screen recordings

How to set up and validate locally

1. Open GraphQL Explorer at http://localhost:3000/-/graphql-explorer
2. Run the following query (replace the project path):

   {
     project(fullPath: "gitlab-org/gitlab-shell") {
       repository {
         commits(ref: "master", first: 10) {
           nodes {
             shortId
             title
             tags
           }
         }
       }
     }
   }

3. Remove tags from the query and verify in Rails logs that no Gitaly:ListRefs call is made

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