Fix bug where the merge description wasn't showing with merge train

Issue: gitlab.com/gitlab-org/gitlab/-/issues/232957
MR: gitlab.com/gitlab-org/gitlab/-/merge_requests/57787

The issue was that the params were strings, { "test" => .. }
when the commit_message was looking for a symbol.

changelogs/unreleased/232957-include-merge-request-description-or-any-custom-merge-commit-messa.yml

+---
+title: Fix issue where merge description not showing when merged with merge train
+merge_request: 57787
+author:
+type: fixed

ee/app/services/merge_trains/refresh_merge_request_service.rb

@@ -62,7 +62,7 @@ module MergeTrains
     def merge!
       merge_train.start_merge!
 
-      MergeRequests::MergeService.new(project, merge_user, merge_request.merge_params)
+      MergeRequests::MergeService.new(project, merge_user, merge_request.merge_params.with_indifferent_access)
                                  .execute(merge_request, skip_discussions_check: true)
 
       raise ProcessError, "failed to merge. #{merge_request.merge_error}" unless merge_request.merged?

ee/spec/services/merge_trains/refresh_merge_request_service_spec.rb

@@ -208,7 +208,7 @@ RSpec.describe MergeTrains::RefreshMergeRequestService do
           expect(merge_request).to receive(:cleanup_refs).with(only: :train)
           expect(merge_request.merge_train).to receive(:start_merge!).and_call_original
           expect(merge_request.merge_train).to receive(:finish_merge!).and_call_original
-          expect_next_instance_of(MergeRequests::MergeService, project, maintainer, anything) do |service|
+          expect_next_instance_of(MergeRequests::MergeService, project, maintainer, instance_of(HashWithIndifferentAccess)) do |service|
             expect(service).to receive(:execute).with(merge_request, skip_discussions_check: true).and_call_original
           end