Add max_pinned_statuses to instances serializer and api response (#29441)

spec/serializers/rest/instance_serializer_spec.rb

@@ -10,11 +10,22 @@ describe REST::InstanceSerializer do
     it 'returns recent usage data' do
       expect(serialization['usage']).to eq({ 'users' => { 'active_month' => 0 } })
     end
+  end
 
+  describe 'configuration' do
     it 'returns the VAPID public key' do
       expect(serialization['configuration']['vapid']).to eq({
         'public_key' => Rails.configuration.x.vapid_public_key,
       })
     end
+
+    it 'returns the max pinned statuses limit' do
+      expect(serialization.deep_symbolize_keys)
+        .to include(
+          configuration: include(
+            accounts: include(max_pinned_statuses: StatusPinValidator::PIN_LIMIT)
+          )
+        )
+    end
   end
 end