feat: add Coupon base class

tests/Unit/Coupons/CouponTest.php

@@ -0,0 +1,72 @@
+<?php
+
+declare(strict_types=1);
+
+use Wearepixel\Cart\CartCondition;
+use Wearepixel\Cart\Coupons\Coupon;
+use Wearepixel\Cart\Exceptions\InvalidCouponException;
+
+class AlwaysValidCoupon extends Coupon
+{
+    protected string $code = 'SAVE10';
+    protected string $value = '-10%';
+
+    public function isValid(): bool
+    {
+        return true;
+    }
+}
+
+class NeverValidCoupon extends Coupon
+{
+    protected string $code = 'EXPIRED';
+    protected string $value = '-10%';
+
+    public function isValid(): bool
+    {
+        return false;
+    }
+}
+
+describe('Coupon', function () {
+    test('converts to a CartCondition', function () {
+        $condition = (new AlwaysValidCoupon)->toCondition();
+
+        expect($condition)->toBeInstanceOf(CartCondition::class);
+        expect($condition->getName())->toBe('SAVE10');
+        expect($condition->getValue())->toBe('-10%');
+        expect($condition->getType())->toBe('coupon');
+        expect($condition->getTarget())->toBe('subtotal');
+    });
+
+    test('respects custom target', function () {
+        $coupon = new class extends Coupon {
+            protected string $code = 'TOTAL10';
+            protected string $value = '-10%';
+            protected string $target = 'total';
+            public function isValid(): bool { return true; }
+        };
+
+        expect($coupon->toCondition()->getTarget())->toBe('total');
+    });
+
+    test('includes minimum in condition when set', function () {
+        $coupon = new class extends Coupon {
+            protected string $code = 'BIG10';
+            protected string $value = '-10%';
+            protected ?float $minimum = 100.0;
+            public function isValid(): bool { return true; }
+        };
+
+        expect($coupon->toCondition()->getMinimum())->toBe(100.0);
+    });
+
+    test('isValid() returning false causes toCondition() to throw', function () {
+        expect(fn() => (new NeverValidCoupon)->toCondition())
+            ->toThrow(InvalidCouponException::class);
+    });
+
+    test('getCode returns the coupon code', function () {
+        expect((new AlwaysValidCoupon)->getCode())->toBe('SAVE10');
+    });
+});