Add state of merge request diff to the entity

This will be used by the frontend to show the banner of the overflow

MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64195

Changelog: added

app/serializers/merge_request_diff_entity.rb

@@ -16,6 +16,7 @@ class MergeRequestDiffEntity < Grape::Entity
   end
   expose :created_at
+  expose :state
   expose :commits_count
   expose :latest?, as: :latest

spec/serializers/merge_request_diff_entity_spec.rb

@@ -29,7 +29,7 @@ def initialize_entity(merge_request, merge_request_diff)
       expect(subject).to include(
         :version_index, :created_at, :commits_count,
         :latest, :short_commit_sha, :version_path,
-        :compare_path
+        :compare_path, :state
       )
     end
   end