feat(wpmc S7): vendor Plugin Update Checker v5.6

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/UpdateCheckerIntegrationTest.php

@@ -0,0 +1,26 @@
+<?php
+declare(strict_types=1);
+
+namespace WPMultiCity\Tests;
+
+use PHPUnit\Framework\TestCase as BaseTestCase;
+
+final class UpdateCheckerIntegrationTest extends BaseTestCase
+{
+    private const PLUGIN_ROOT = __DIR__ . '/..';
+    private const PUC_LOADER  = self::PLUGIN_ROOT . '/includes/lib/plugin-update-checker/plugin-update-checker.php';
+
+    public function test_puc_loader_exists(): void
+    {
+        self::assertFileExists(self::PUC_LOADER);
+    }
+
+    public function test_puc_factory_loadable(): void
+    {
+        require_once self::PUC_LOADER;
+        self::assertTrue(
+            class_exists('YahnisElsts\\PluginUpdateChecker\\v5\\PucFactory'),
+            'PucFactory must be loadable after requiring the PUC loader'
+        );
+    }
+}