Add missing policy attribute to WebPushSubscriptionSerializer (#23210)

* Add missing `policy` attribute to `WebPushSubscriptionSerializer`

Fixes #23145

* Add tests

spec/controllers/api/v1/push/subscriptions_controller_spec.rb

@@ -61,6 +61,10 @@ describe Api::V1::Push::SubscriptionsController do
       post :create, params: create_payload
       expect(Web::PushSubscription.where(endpoint: create_payload[:subscription][:endpoint]).count).to eq 1
     end
+
+    it 'returns the expected JSON' do
+      expect(body_as_json.with_indifferent_access).to include({ endpoint: create_payload[:subscription][:endpoint], alerts: {}, policy: 'all' })
+    end
   end
 
   describe 'PUT #update' do
@@ -78,6 +82,10 @@ describe Api::V1::Push::SubscriptionsController do
         expect(push_subscription.data['alerts'][type]).to eq(alerts_payload[:data][:alerts][type.to_sym].to_s)
       end
     end
+
+    it 'returns the expected JSON' do
+      expect(body_as_json.with_indifferent_access).to include({ endpoint: create_payload[:subscription][:endpoint], alerts: alerts_payload[:data][:alerts], policy: alerts_payload[:data][:policy] })
+    end
   end
 
   describe 'DELETE #destroy' do