feat: add TaxRule base class

tests/Unit/Tax/TaxRuleTest.php

@@ -0,0 +1,68 @@
+<?php
+
+declare(strict_types=1);
+
+use Wearepixel\Cart\CartCondition;
+use Wearepixel\Cart\Tax\TaxRule;
+use Wearepixel\Cart\Exceptions\InvalidTaxRuleException;
+
+class AuGst extends TaxRule
+{
+    protected string $name = 'GST';
+    protected string $value = '10%';
+
+    public function isApplicable(): bool
+    {
+        return true;
+    }
+}
+
+class NotApplicableTax extends TaxRule
+{
+    protected string $name = 'VAT';
+    protected string $value = '20%';
+
+    public function isApplicable(): bool
+    {
+        return false;
+    }
+}
+
+describe('TaxRule', function () {
+    test('converts to a CartCondition', function () {
+        $condition = (new AuGst)->toCondition();
+
+        expect($condition)->toBeInstanceOf(CartCondition::class);
+        expect($condition->getName())->toBe('GST');
+        expect($condition->getValue())->toBe('10%');
+        expect($condition->getType())->toBe('tax');
+        expect($condition->getTarget())->toBe('subtotal');
+    });
+
+    test('isApplicable() returning false causes toCondition() to throw', function () {
+        expect(fn() => (new NotApplicableTax)->toCondition())
+            ->toThrow(InvalidTaxRuleException::class);
+    });
+
+    test('supports total as target', function () {
+        $tax = new class extends TaxRule {
+            protected string $name = 'Sales Tax';
+            protected string $value = '8%';
+            protected string $target = 'total';
+            public function isApplicable(): bool { return true; }
+        };
+
+        expect($tax->toCondition()->getTarget())->toBe('total');
+    });
+
+    test('supports minimum threshold', function () {
+        $tax = new class extends TaxRule {
+            protected string $name = 'Luxury Tax';
+            protected string $value = '15%';
+            protected ?float $minimum = 500.0;
+            public function isApplicable(): bool { return true; }
+        };
+
+        expect($tax->toCondition()->getMinimum())->toBe(500.0);
+    });
+});