Add explicit arg check in :minutes_cost_factor

ee/app/models/ee/ci/runner.rb

@@ -15,8 +15,10 @@ module EE
         case access_level
         when ::Gitlab::VisibilityLevel::PUBLIC
           public_projects_minutes_cost_factor
-        else # Gitlab::VisibilityLevel::PRIVATE/INTERNAL
+        when ::Gitlab::VisibilityLevel::PRIVATE, ::Gitlab::VisibilityLevel::INTERNAL
           private_projects_minutes_cost_factor
+        else
+          raise ArgumentError, 'Invalid visibility level'
         end
       end
 

ee/spec/models/ee/ci/runner_spec.rb

@@ -71,6 +71,14 @@ describe EE::Ci::Runner do
 
         it { is_expected.to eq(1.1) }
       end
+
+      context 'with invalid visibility level' do
+        let(:visibility_level) { 123 }
+
+        it 'raises an error' do
+          expect { subject }.to raise_error(ArgumentError)
+        end
+      end
     end
   end