Add PHPUnit test suite, PSR-4 refactor, lint+test CI jobs (v1.3.1)

- 57 unit tests covering ProductType, StockManager, CartHandler, Plugin,
  Admin/ProductData, Admin/Settings using Brain Monkey + Mockery
- WooCommerce class stubs for testing without WP installation
- PHP lint and test jobs in release workflow (test gate blocks release)
- PSR-4 namespace change: WC_Composable_Product -> Magdev\WcComposableProduct
- PascalCase filenames for all classes under includes/

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/TestCase.php

@@ -0,0 +1,75 @@
+<?php
+/**
+ * Base Test Case
+ *
+ * @package Magdev\WcComposableProduct\Tests
+ */
+
+namespace Magdev\WcComposableProduct\Tests;
+
+use PHPUnit\Framework\TestCase as PHPUnitTestCase;
+use Mockery\Adapter\Phpunit\MockeryPHPUnitIntegration;
+use Brain\Monkey;
+
+/**
+ * Base test case with Brain Monkey and Mockery integration.
+ *
+ * All test classes should extend this instead of PHPUnit\Framework\TestCase.
+ */
+abstract class TestCase extends PHPUnitTestCase
+{
+    use MockeryPHPUnitIntegration;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+        Monkey\setUp();
+
+        // Stub common WordPress translation functions (__(), _e(), esc_html__(), etc.)
+        Monkey\Functions\stubTranslationFunctions();
+
+        // Stub common WordPress escaping functions (esc_html(), esc_attr(), etc.)
+        Monkey\Functions\stubEscapeFunctions();
+    }
+
+    protected function tearDown(): void
+    {
+        Monkey\tearDown();
+        parent::tearDown();
+    }
+
+    /**
+     * Create a Mockery mock of WC_Product with sensible defaults.
+     *
+     * @param array $overrides Method return value overrides
+     * @return \Mockery\MockInterface
+     */
+    protected function createProductMock(array $overrides = []): \Mockery\MockInterface
+    {
+        $defaults = [
+            'get_id' => 100,
+            'get_name' => 'Test Product',
+            'get_type' => 'simple',
+            'get_price' => '10.00',
+            'get_regular_price' => '10.00',
+            'get_price_html' => '<span>$10.00</span>',
+            'get_permalink' => 'https://example.com/product/test',
+            'get_image_id' => 1,
+            'get_stock_quantity' => null,
+            'get_stock_status' => 'instock',
+            'is_purchasable' => true,
+            'is_in_stock' => true,
+            'managing_stock' => false,
+            'backorders_allowed' => false,
+        ];
+
+        $config = array_merge($defaults, $overrides);
+        $mock = \Mockery::mock('WC_Product');
+
+        foreach ($config as $method => $return) {
+            $mock->shouldReceive($method)->andReturn($return)->byDefault();
+        }
+
+        return $mock;
+    }
+}