test: add withCoupon test and fix withItems ID collision

src/Testing/CartFactory.php

@@ -14,7 +14,8 @@ public function __construct(private Cart $cart) {}
 
     public function withItems(int $count): static
     {
-        for ($i = 1; $i <= $count; $i++) {
+        $start = $this->cart->getContent()->count() + 1;
+        for ($i = $start; $i < $start + $count; $i++) {
             $this->cart->add($i, "Test Item {$i}", 10.00, 1);
         }
 

tests/Unit/Testing/CartFactoryTest.php

@@ -57,4 +57,18 @@ function makeCartForFactory(): Cart
 
         expect(array_unique($ids))->toHaveCount(3);
     });
+
+    test('withCoupon adds a coupon condition to the cart', function () {
+        $coupon = new class extends \Wearepixel\Cart\Coupons\Coupon {
+            protected string $code = 'SAVE10';
+            protected string $value = '-10%';
+            protected string $target = 'subtotal';
+
+            public function isValid(): bool { return true; }
+        };
+
+        $this->factory->withCoupon($coupon);
+
+        expect($this->cart->getCondition('SAVE10'))->not->toBeNull();
+    });
 });