Fix AutoMergeProcessWorker failing if merge train service isnt available

This change fixes AutoMergeProcessWorker from failing when
merge train service is not available for a merge request

changelogs/unreleased/198042-argumenterror-wrong-number-of-arguments-given-1-expected-2.yml

+---
+title: Fixes AutoMergeProcessWorker failing when merge train service is not available
+  for a merge request
+merge_request: 23407
+author:
+type: fixed

ee/app/services/auto_merge/add_to_merge_train_when_pipeline_succeeds_service.rb

@@ -13,7 +13,7 @@ module AutoMerge
 
       merge_train_service = AutoMerge::MergeTrainService.new(project, merge_request.merge_user)
 
-      return abort(merge_request) unless merge_train_service.available_for?(merge_request)
+      return abort(merge_request, 'This merge request cannot be added to the merge train') unless merge_train_service.available_for?(merge_request)
 
       merge_train_service.execute(merge_request)
     end

ee/spec/services/auto_merge/add_to_merge_train_when_pipeline_succeeds_service_spec.rb

@@ -61,7 +61,11 @@ describe AutoMerge::AddToMergeTrainWhenPipelineSucceedsService do
         end
 
         it 'aborts auto merge' do
-          expect(service).to receive(:abort).once
+          expect(service).to receive(:abort).once.and_call_original
+
+          expect(SystemNoteService)
+            .to receive(:abort_add_to_merge_train_when_pipeline_succeeds).once
+            .with(merge_request, project, user, 'This merge request cannot be added to the merge train')
 
           subject
         end