feat(products): expose product_items_count on the product API

## Context

The product catalog needs the number of items per product, but returning
the full items collection inline would bloat the payload. The GraphQL type
already exposes product_items_count only.

## Description

Add product_items_count to the REST product serializer to match the
GraphQL shape, without serializing the items themselves.

Author: rsempe

app/serializers/v1/product_serializer.rb

@@ -9,6 +9,7 @@ def serialize
         code: model.code,
         description: model.description,
         invoice_display_name: model.invoice_display_name,
+        product_items_count: model.product_items.count,
         created_at: model.created_at.iso8601
       }
     end

spec/requests/api/v1/products_controller_spec.rb

@@ -76,11 +76,14 @@
     include_examples "requires API permission", "product", "read"
 
     it "returns the product" do
+      create(:product_item, organization:, product:)
+
       subject
 
       expect(response).to have_http_status(:success)
       expect(json[:product][:lago_id]).to eq(product.id)
       expect(json[:product][:code]).to eq(product.code)
+      expect(json[:product][:product_items_count]).to eq(1)
     end
 
     context "when the product does not exist" do