feat: add ShippingRate base class

tests/Unit/Shipping/ShippingRateTest.php

@@ -0,0 +1,68 @@
+<?php
+
+declare(strict_types=1);
+
+use Wearepixel\Cart\CartCondition;
+use Wearepixel\Cart\Shipping\ShippingRate;
+use Wearepixel\Cart\Exceptions\InvalidShippingRateException;
+
+class FlatRateShipping extends ShippingRate
+{
+    protected string $name = 'Standard Shipping';
+    protected string $value = '+10';
+
+    public function isApplicable(): bool
+    {
+        return true;
+    }
+}
+
+class FreeShipping extends ShippingRate
+{
+    protected string $name = 'Free Shipping';
+    protected string $value = '+0';
+
+    public function isApplicable(): bool
+    {
+        return false;
+    }
+}
+
+describe('ShippingRate', function () {
+    test('converts to a CartCondition', function () {
+        $condition = (new FlatRateShipping)->toCondition();
+
+        expect($condition)->toBeInstanceOf(CartCondition::class);
+        expect($condition->getName())->toBe('Standard Shipping');
+        expect($condition->getValue())->toBe('+10');
+        expect($condition->getType())->toBe('shipping');
+        expect($condition->getTarget())->toBe('total');
+    });
+
+    test('isApplicable() returning false causes toCondition() to throw', function () {
+        expect(fn() => (new FreeShipping)->toCondition())
+            ->toThrow(InvalidShippingRateException::class);
+    });
+
+    test('supports subtotal as target', function () {
+        $rate = new class extends ShippingRate {
+            protected string $name = 'Express';
+            protected string $value = '+25';
+            protected string $target = 'subtotal';
+            public function isApplicable(): bool { return true; }
+        };
+
+        expect($rate->toCondition()->getTarget())->toBe('subtotal');
+    });
+
+    test('supports maximum free shipping threshold', function () {
+        $rate = new class extends ShippingRate {
+            protected string $name = 'Conditional Shipping';
+            protected string $value = '+15';
+            protected ?float $maximum = 100.0;
+            public function isApplicable(): bool { return true; }
+        };
+
+        expect($rate->toCondition()->getMaximum())->toBe(100.0);
+    });
+});