1. Observed:  POST /orders returns 500 when product is out of stock
2. Expected:  Returns 422 with { error: "Out of stock" }
3. Boundary:  Request layer — visible in HTTP contract
4. First spec: spec/requests/orders/create_spec.rb
5. Fix path:  Orders::CreateOrder should return { success: false, error: "Out of stock" }
              when inventory check fails; controller renders 422
6. Next:      rails-tdd-slices → write request spec → rspec-best-practices → fix

# spec/requests/orders/create_spec.rb
RSpec.describe "POST /orders", type: :request do
  context "when product is out of stock" do
    let(:product) { create(:product, stock: 0) }

    it "returns 422 with an error message" do
      post orders_path, params: { order: { product_id: product.id, quantity: 1 } }, as: :json
      expect(response).to have_http_status(:unprocessable_entity)
      expect(response.parsed_body["error"]).to eq("Out of stock")
    end
  end
end