Refactor project API helpers

What does this MR do?

Coming from #37919 (closed), this MR

1. Adds a regression spec for this type of problem
2. Refactors the projects API (helper) methods to make side effects more explicit.

In more detail, the side effect I wanted to make more explicit is the fact that we perform batch counting to refresh caches. This had been mixed into preparing the database query to retrieve projects (hence I think of it as a "side effect") and didn't have specs.

Closes #37919 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• This is a refactoring only, but we've run into missing tests before.

lib/api/entities.rb

@@ -211,14 +211,6 @@ class BasicProjectDetails < ProjectIdentity
       expose :namespace, using: 'API::Entities::NamespaceBasic'
       expose :custom_attributes, using: 'API::Entities::CustomAttribute', if: :with_custom_attributes
 
-      # This adds preloading to the query and executes batch counting
-      # Side-effect: The query will be executed during batch counting
-      def self.prepare!(projects_relation)
-        preload_relation(projects_relation).tap do |projects|
-          execute_batch_counting(projects)
-        end
-      end
-
       # rubocop: disable CodeReuse/ActiveRecord
       def self.preload_relation(projects_relation, options = {})
         # Preloading tags, should be done with using only `:tags`,