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/FakeCartTest.php

@@ -0,0 +1,112 @@
+<?php
+
+use Wearepixel\Cart\Cart;
+use Wearepixel\Cart\CartCondition;
+use Wearepixel\Cart\Drivers\SessionDriver;
+use Wearepixel\Cart\Tests\Helpers\SessionMock;
+
+function makeFakeableCart(): 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 () {
+    $this->cart = makeFakeableCart();
+});
+
+afterEach(fn() => Mockery::close());
+
+describe('Cart::fake()', function () {
+    test('returns the CartFactory instance', function () {
+        expect($this->cart->fake())->toBeInstanceOf(\Wearepixel\Cart\Testing\CartFactory::class);
+    });
+
+    test('swaps driver to NullDriver', function () {
+        $this->cart->fake();
+        expect($this->cart->getDriver()->getDriverName())->toBe('null');
+    });
+
+    test('Cart operations work after fake()', function () {
+        $this->cart->fake();
+        $this->cart->add(1, 'Widget', 50.00, 2);
+
+        expect($this->cart->isEmpty())->toBeFalse();
+        expect($this->cart->getContent()->count())->toBe(1);
+    });
+
+    test('assertContains passes when item is in cart', function () {
+        $this->cart->fake();
+        $this->cart->add(1, 'Widget', 50.00, 1);
+
+        expect(fn() => $this->cart->assertContains(1))->not->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertContains fails when item is not in cart', function () {
+        $this->cart->fake();
+
+        expect(fn() => $this->cart->assertContains(99))->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertCount passes with correct count', function () {
+        $this->cart->fake();
+        $this->cart->add(1, 'A', 10.00, 1);
+        $this->cart->add(2, 'B', 20.00, 1);
+
+        expect(fn() => $this->cart->assertCount(2))->not->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertTotalQuantity passes with correct quantity', function () {
+        $this->cart->fake();
+        $this->cart->add(1, 'A', 10.00, 3);
+
+        expect(fn() => $this->cart->assertTotalQuantity(3))->not->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertSubTotal passes with correct subtotal', function () {
+        $this->cart->fake();
+        $this->cart->add(1, 'Widget', 50.00, 2);
+
+        expect(fn() => $this->cart->assertSubTotal(100.00))->not->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertTotal passes with correct total', function () {
+        $this->cart->fake();
+        $this->cart->add(1, 'Widget', 50.00, 1);
+
+        expect(fn() => $this->cart->assertTotal(50.00))->not->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertConditionApplied passes when condition exists', function () {
+        $this->cart->fake();
+        $this->cart->condition(new CartCondition(['name' => 'GST', 'type' => 'tax', 'value' => '10%', 'target' => 'subtotal']));
+
+        expect(fn() => $this->cart->assertConditionApplied('GST'))->not->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertConditionApplied fails when condition does not exist', function () {
+        $this->cart->fake();
+
+        expect(fn() => $this->cart->assertConditionApplied('GST'))->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertEmpty passes when cart is empty', function () {
+        $this->cart->fake();
+
+        expect(fn() => $this->cart->assertEmpty())->not->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+
+    test('assertNotEmpty passes when cart has items', function () {
+        $this->cart->fake();
+        $this->cart->add(1, 'Widget', 10.00, 1);
+
+        expect(fn() => $this->cart->assertNotEmpty())->not->toThrow(\PHPUnit\Framework\AssertionFailedError::class);
+    });
+});