Add lookahead preloading for cluster agent tokens

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40779

ee/app/graphql/ee/types/project_type.rb

@@ -101,6 +101,7 @@ module EE
 
         field :cluster_agents,
               ::Types::Clusters::AgentType.connection_type,
+              extras: [:lookahead],
               null: true,
               description: 'Cluster agents associated with the project',
               resolver: ::Resolvers::Clusters::AgentsResolver

ee/app/graphql/resolvers/clusters/agents_resolver.rb

@@ -3,14 +3,24 @@
 module Resolvers
   module Clusters
     class AgentsResolver < BaseResolver
+      include LooksAhead
+
       type Types::Clusters::AgentType, null: true
 
       alias_method :project, :object
 
-      def resolve(**args)
+      def resolve_with_lookahead(**args)
+        apply_lookahead(
           ::Clusters::AgentsFinder
             .new(project, context[:current_user], params: args)
             .execute
+        )
+      end
+
+      private
+
+      def preloads
+        { tokens: :agent_tokens }
       end
     end
   end

ee/spec/graphql/resolvers/clusters/agents_resolver_spec.rb

@@ -5,24 +5,36 @@ require 'spec_helper'
 RSpec.describe Resolvers::Clusters::AgentsResolver do
   include GraphqlHelpers
 
+  it { expect(described_class).to be < LooksAhead }
+
   it { expect(described_class.type).to eq(Types::Clusters::AgentType) }
   it { expect(described_class.null).to be_truthy }
 
   describe '#resolve' do
     let_it_be(:user) { create(:user) }
 
-    let(:finder) { double(execute: :result) }
+    let(:finder) { double(execute: relation) }
+    let(:relation) { double }
     let(:project) { create(:project) }
     let(:args) { Hash(key: 'value') }
     let(:ctx) { Hash(current_user: user) }
 
-    subject { resolve(described_class, obj: project, args: args, ctx: ctx) }
+    let(:lookahead) do
+      double(selects?: true).tap do |selection|
+        allow(selection).to receive(:selection).and_return(selection)
+      end
+    end
+
+    subject { resolve(described_class, obj: project, args: args.merge(lookahead: lookahead), ctx: ctx) }
 
     it 'calls the agents finder' do
       expect(::Clusters::AgentsFinder).to receive(:new)
         .with(project, user, params: args).and_return(finder)
 
-      expect(subject).to eq(:result)
+      expect(relation).to receive(:preload)
+        .with(:agent_tokens).and_return(relation)
+
+      expect(subject).to eq(relation)
     end
   end
 end