feat: add Cart::fake(), assertion methods, and CartFactory

Adds Cart::fake() which swaps the active driver to NullDriver and returns
a CartFactory fluent test builder. Adds assertion methods (assertContains,
assertCount, assertTotalQuantity, assertSubTotal, assertTotal,
assertConditionApplied, assertEmpty, assertNotEmpty) to Cart for use in
tests. CartFactory provides withItems(), withCondition(), and withCoupon()
as chainable helpers for seeding cart state.

tests/Unit/Testing/CartFactoryTest.php

@@ -0,0 +1,60 @@
+<?php
+
+use Wearepixel\Cart\Cart;
+use Wearepixel\Cart\CartCondition;
+use Wearepixel\Cart\Drivers\SessionDriver;
+use Wearepixel\Cart\Testing\CartFactory;
+use Wearepixel\Cart\Tests\Helpers\SessionMock;
+
+function makeCartForFactory(): Cart
+{
+    $events = Mockery::mock('Illuminate\Contracts\Events\Dispatcher');
+    $events->shouldReceive('dispatch');
+
+    return new Cart(
+        new SessionDriver(new SessionMock, 'TESTKEY'),
+        $events,
+        'cart',
+        require __DIR__ . '/../../Helpers/ConfigMock.php',
+    );
+}
+
+beforeEach(function () {
+    $this->cart = makeCartForFactory();
+    $this->factory = new CartFactory($this->cart);
+});
+
+afterEach(fn() => Mockery::close());
+
+describe('CartFactory', function () {
+    test('withItems adds N generic items to the cart', function () {
+        $this->factory->withItems(3);
+
+        expect($this->cart->getContent()->count())->toBe(3);
+        expect($this->cart->getTotalQuantity())->toBe(3);
+    });
+
+    test('withCondition adds a condition to the cart', function () {
+        $condition = new CartCondition(['name' => 'GST', 'type' => 'tax', 'value' => '10%', 'target' => 'subtotal']);
+        $this->factory->withCondition($condition);
+
+        expect($this->cart->getCondition('GST'))->not->toBeNull();
+    });
+
+    test('methods are chainable', function () {
+        $result = $this->factory->withItems(2)->withCondition(
+            new CartCondition(['name' => 'Discount', 'type' => 'discount', 'value' => '-5%', 'target' => 'subtotal'])
+        );
+
+        expect($result)->toBeInstanceOf(CartFactory::class);
+        expect($this->cart->getContent()->count())->toBe(2);
+        expect($this->cart->getCondition('Discount'))->not->toBeNull();
+    });
+
+    test('withItems generates unique item IDs', function () {
+        $this->factory->withItems(3);
+        $ids = $this->cart->getContent()->keys()->toArray();
+
+        expect(array_unique($ids))->toHaveCount(3);
+    });
+});