Issue #3375880: Fix CKEditor5 library dependency.

Closes #3375880

tests/src/Kernel/MaxLengthCkeditorTest.php

+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\maxlength\Kernel;
+
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Provides kernel tests MaxLength with CKEditor5.
+ *
+ * @group maxlength
+ */
+class MaxLengthCkeditorTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'maxlength',
+    'field',
+    'user',
+    'system',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->installConfig(['maxlength']);
+  }
+
+  /**
+   * Tests the CKEditor5 library dependency.
+   *
+   * @covers maxlength_library_info_alter()
+   */
+  public function testCkeditor5LibraryDependency() {
+    // Get the library discovery service.
+    /** @var \Drupal\Core\Asset\LibraryDiscovery $libraryDiscovery */
+    $library_discovery = $this->container->get('library.discovery');
+    $maxlength_library = $library_discovery->getLibraryByName('maxlength', 'maxlength');
+    $this->assertNotContains('ckeditor5/internal.drupal.ckeditor5', $maxlength_library['dependencies']);
+
+    // Now enable CKEditor5 module.
+    $this->container->get('module_installer')->install(['ckeditor5']);
+    $library_discovery->clearCachedDefinitions();
+    $maxlength_library = $library_discovery->getLibraryByName('maxlength', 'maxlength');
+    $this->assertContains('ckeditor5/internal.drupal.ckeditor5', $maxlength_library['dependencies']);
+  }
+
+}