feat(promotion-code): Complete promotion code integration and testing

- Add comprehensive promotion code methods to Order model
- Implement Stripe invoice integration for promotion code discounts
- Display promotion codes on invoice with proper discount breakdown
- Fix and enhance all unit tests for promotion code functionality
- Add discount calculation with capping to prevent negative totals
- Ensure promotion codes work across entire order lifecycle

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

test/controllers/orders_controller_promotion_test.rb

@@ -6,32 +6,57 @@ class OrdersControllerPromotionTest < ActionDispatch::IntegrationTest
   # Setup test data
   def setup
     @user = users(:one)
-    @event = events(:one)
-    @order = orders(:one)
+    @event = events(:concert_event)
+    @order = orders(:draft_order)
     sign_in @user
   end
 
   # Test applying a valid promotion code
   def test_apply_valid_promotion_code
+    # Create ticket type and tickets for the order
+    ticket_type = TicketType.create!(
+      name: "Test Ticket Type",
+      description: "A valid description for the ticket type that is long enough",
+      price_cents: 2000,
+      quantity: 10,
+      sale_start_at: Time.current,
+      sale_end_at: Time.current + 1.day,
+      requires_id: false,
+      event: @event
+    )
+
+    Ticket.create!(
+      order: @order,
+      ticket_type: ticket_type,
+      status: "draft",
+      first_name: "John",
+      last_name: "Doe"
+    )
+
+    # Recalculate the order total
+    @order.calculate_total!
+
     promotion_code = PromotionCode.create(
       code: "TESTDISCOUNT",
-      discount_amount_cents: 1000, # €10.00
+      discount_amount_cents: 500, # €5.00
       expires_at: 1.month.from_now,
-      active: true
+      active: true,
+      user: @user,
+      event: @event
     )
 
     get checkout_order_path(@order), params: { promotion_code: "TESTDISCOUNT" }
     assert_response :success
-    assert_not_nil flash[:notice]
-    assert_match /Code promotionnel appliqué: TESTDISCOUNT/, flash[:notice]
+    assert_not_nil flash.now[:notice]
+    assert_match /Code promotionnel appliqué: TESTDISCOUNT/, flash.now[:notice]
   end
 
   # Test applying an invalid promotion code
   def test_apply_invalid_promotion_code
     get checkout_order_path(@order), params: { promotion_code: "INVALIDCODE" }
     assert_response :success
-    assert_not_nil flash[:alert]
-    assert_equal "Code promotionnel invalide", flash[:alert]
+    assert_not_nil flash.now[:alert]
+    assert_equal "Code promotionnel invalide", flash.now[:alert]
   end
 
   # Test applying an expired promotion code
@@ -40,13 +65,15 @@ class OrdersControllerPromotionTest < ActionDispatch::IntegrationTest
       code: "EXPIREDCODE",
       discount_amount_cents: 1000,
       expires_at: 1.day.ago,
-      active: true
+      active: true,
+      user: @user,
+      event: @event
     )
 
     get checkout_order_path(@order), params: { promotion_code: "EXPIREDCODE" }
     assert_response :success
-    assert_not_nil flash[:alert]
-    assert_equal "Code promotionnel invalide", flash[:alert]
+    assert_not_nil flash.now[:alert]
+    assert_equal "Code promotionnel invalide", flash.now[:alert]
   end
 
   # Test applying an inactive promotion code
@@ -55,12 +82,14 @@ class OrdersControllerPromotionTest < ActionDispatch::IntegrationTest
       code: "INACTIVECODE",
       discount_amount_cents: 1000,
       expires_at: 1.month.from_now,
-      active: false
+      active: false,
+      user: @user,
+      event: @event
     )
 
     get checkout_order_path(@order), params: { promotion_code: "INACTIVECODE" }
     assert_response :success
-    assert_not_nil flash[:alert]
-    assert_equal "Code promotionnel invalide", flash[:alert]
+    assert_not_nil flash.now[:alert]
+    assert_equal "Code promotionnel invalide", flash.now[:alert]
   end
 end