Do not cast project IDs to string in API response

Previously we were casting the invalid project IDs to strings because
we were trying to cover the case where the client sends an invalid ID
to the backend. This was causing problems on client application because
it was expecting the IDs to be integers. I do believe that the effort
of covering the case where client sends an invalid ID will cause more
problems than it solves therefore I am removing the related logic and
assuming the client is always sending us valid IDs.

/closes https://gitlab.com/gitlab-org/gitlab/-/issues/196729

ee/app/services/dashboard/projects/create_service.rb

@@ -31,9 +31,9 @@ module Dashboard
       end
 
       def find_invalid_ids(projects_to_add, project_ids)
-        found_ids = projects_to_add.map(&:id).map(&:to_s)
+        found_ids = projects_to_add.map(&:id)
 
-        project_ids.map(&:to_s) - found_ids
+        project_ids.map(&:to_i) - found_ids
       end
 
       def add_projects(projects)

ee/spec/controllers/operations_controller_spec.rb

@@ -722,7 +722,7 @@ describe OperationsController do
         expect(json_response).to match_schema('dashboard/operations/add', dir: 'ee')
         expect(json_response['added']).to be_empty
         expect(json_response['duplicate']).to be_empty
-        expect(json_response['invalid']).to contain_exactly('', '-1', '-2')
+        expect(json_response['invalid']).to contain_exactly(0, -1, -2)
 
         user.reload
         expect(user.ops_dashboard_projects).to be_empty

ee/spec/controllers/security/projects_controller_spec.rb

@@ -107,7 +107,7 @@ describe Security::ProjectsController do
             expect(json_response).to eq({
               'added' => [],
               'duplicate' => [],
-              'invalid' => ['-1']
+              'invalid' => [-1]
             })
           end
         end

ee/spec/fixtures/api/schemas/dashboard/operations/add.json

@@ -17,10 +17,7 @@
     "invalid": {
       "type": "array",
       "items": {
-        "oneOf": [
-          { "type": "string" },
-          { "type": "null" }
-        ]
+        "items": { "type": "integer" }
       }
     }
   },

ee/spec/services/dashboard/projects/create_service_spec.rb

@@ -57,15 +57,6 @@ describe Dashboard::Projects::CreateService do
         end
       end
     end
-
-    context 'with invalid project ids' do
-      let(:input) { [nil, -1, '-1', :symbol] }
-      let(:output) { [] }
-
-      it 'does not add invalid project ids' do
-        expect(result).to eq(expected_result(invalid_project_ids: input.map(&:to_s)))
-      end
-    end
   end
 
   private