Fix namespacing for MergeWhenPipelineSucceedsService in MR API

changelogs/unreleased/39619-cancel-merge-when-pipeline-succeeds-from-the-api-fails.yml

+---
+title: Fix namespacing for MergeWhenPipelineSucceedsService in MR API
+merge_request:
+author:
+type: fixed

lib/api/merge_requests.rb

@@ -295,7 +295,7 @@ module API
 
         unauthorized! unless merge_request.can_cancel_merge_when_pipeline_succeeds?(current_user)
 
-        ::MergeRequest::MergeWhenPipelineSucceedsService
+        ::MergeRequests::MergeWhenPipelineSucceedsService
           .new(merge_request.target_project, current_user)
           .cancel(merge_request)
       end

spec/requests/api/merge_requests_spec.rb

@@ -1061,6 +1061,30 @@ describe API::MergeRequests do
     end
   end
 
+  describe 'POST :id/merge_requests/:merge_request_iid/cancel_merge_when_pipeline_succeeds' do
+    before do
+      ::MergeRequests::MergeWhenPipelineSucceedsService.new(merge_request.target_project, user).execute(merge_request)
+    end
+
+    it 'removes the merge_when_pipeline_succeeds status' do
+      post api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/cancel_merge_when_pipeline_succeeds", user)
+
+      expect(response).to have_gitlab_http_status(201)
+    end
+
+    it 'returns 404 if the merge request is not found' do
+      post api("/projects/#{project.id}/merge_requests/123/merge_when_pipeline_succeeds", user)
+
+      expect(response).to have_gitlab_http_status(404)
+    end
+
+    it 'returns 404 if the merge request id is used instead of iid' do
+      post api("/projects/#{project.id}/merge_requests/#{merge_request.id}/merge_when_pipeline_succeeds", user)
+
+      expect(response).to have_gitlab_http_status(404)
+    end
+  end
+
   describe 'Time tracking' do
     let(:issuable) { merge_request }