Add PHPUnit testing framework and error checking documentation

tests/phpunit/bootstrap.php

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+<?php
+/**
+ * PHPUnit bootstrap file.
+ *
+ * @package WP_Plugin_Starter_Template_For_AI_Coding
+ */
+
+// Composer autoloader must be loaded before WP_PHPUNIT__DIR will be available.
+require_once dirname( dirname( __DIR__ ) ) . '/vendor/autoload.php';
+
+// Give access to tests_add_filter() function.
+require_once getenv( 'WP_PHPUNIT__DIR' ) . '/includes/functions.php';
+
+/**
+ * Manually load the plugin being tested.
+ */
+function _manually_load_plugin() {
+	require dirname( dirname( __DIR__ ) ) . '/plugin-toggle.php';
+}
+
+// Start up the WP testing environment.
+require getenv( 'WP_PHPUNIT__DIR' ) . '/includes/bootstrap.php';

tests/phpunit/test-multisite.php

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+<?php
+/**
+ * Class MultisiteTest
+ *
+ * @package WP_Plugin_Starter_Template_For_AI_Coding
+ */
+
+/**
+ * Sample test case for the Multisite class.
+ */
+class MultisiteTest extends WP_UnitTestCase {
+
+	/**
+	 * Test instance creation.
+	 */
+	public function test_instance() {
+		$multisite = new WP_Plugin_Starter_Template_For_AI_Coding\Multisite\Multisite();
+		$this->assertInstanceOf( 'WP_Plugin_Starter_Template_For_AI_Coding\Multisite\Multisite', $multisite );
+	}
+
+	/**
+	 * Test is_multisite_compatible method.
+	 */
+	public function test_is_multisite_compatible() {
+		$multisite = new WP_Plugin_Starter_Template_For_AI_Coding\Multisite\Multisite();
+		$this->assertTrue( $multisite->is_multisite_compatible() );
+	}
+
+	/**
+	 * Test get_network_sites method.
+	 */
+	public function test_get_network_sites() {
+		$multisite = new WP_Plugin_Starter_Template_For_AI_Coding\Multisite\Multisite();
+		
+		// Mock the get_sites function if we're not in a multisite environment.
+		if ( ! function_exists( 'get_sites' ) ) {
+			$this->assertEquals( array(), $multisite->get_network_sites() );
+		} else {
+			$sites = $multisite->get_network_sites();
+			$this->assertIsArray( $sites );
+		}
+	}
+
+	/**
+	 * Test initialize_hooks method.
+	 */
+	public function test_initialize_hooks() {
+		$multisite = new WP_Plugin_Starter_Template_For_AI_Coding\Multisite\Multisite();
+		
+		// Call the method.
+		$multisite->initialize_hooks();
+		
+		// Check if the action was added.
+		$this->assertEquals( 10, has_action( 'network_admin_menu', array( $multisite, 'add_network_menu' ) ) );
+	}
+}