chore(06-02): download and include Action Scheduler library

- Downloaded Action Scheduler 3.9.3 from GitHub
- Placed in vendor/action-scheduler/ directory
- Included in main plugin file before other code for proper initialization
- Library will auto-initialize itself when required

vendor/action-scheduler/classes/ActionScheduler_RecurringActionScheduler.php

@@ -0,0 +1,81 @@
+<?php
+
+/**
+ * Class ActionScheduler_RecurringActionScheduler
+ *
+ * This class ensures that the `action_scheduler_ensure_recurring_actions` hook is triggered on a daily interval. This
+ * simplifies the process for other plugins to register their recurring actions without requiring each plugin to query
+ * or schedule actions independently on every request.
+ */
+class ActionScheduler_RecurringActionScheduler {
+
+	/**
+	 * @var string The hook of the scheduled recurring action that is run to trigger the
+	 *      `action_scheduler_ensure_recurring_actions` hook that plugins should use.  We can't directly have the
+	 *      scheduled action hook be the hook plugins should use because the actions will show as failed if no plugin
+	 *      was actively hooked into it.
+	 */
+	private const RUN_SCHEDULED_RECURRING_ACTIONS_HOOK = 'action_scheduler_run_recurring_actions_schedule_hook';
+
+	/**
+	 * Initialize the instance.  Should only be run on a single instance per request.
+	 *
+	 * @return void
+	 */
+	public function init(): void {
+		add_action( self::RUN_SCHEDULED_RECURRING_ACTIONS_HOOK, array( $this, 'run_recurring_scheduler_hook' ) );
+		if ( is_admin() && ( ! defined( 'DOING_AJAX' ) || ! DOING_AJAX ) ) {
+			add_action( 'action_scheduler_init', array( $this, 'schedule_recurring_scheduler_hook' ) );
+		}
+	}
+
+	/**
+	 * Schedule the recurring `action_scheduler_ensure_recurring_actions` action if not already scheduled.
+	 *
+	 * @return void
+	 */
+	public function schedule_recurring_scheduler_hook(): void {
+		if ( false === wp_cache_get( 'as_is_ensure_recurring_actions_scheduled' ) ) {
+			if ( ! as_has_scheduled_action( self::RUN_SCHEDULED_RECURRING_ACTIONS_HOOK ) ) {
+				as_schedule_recurring_action(
+					time(),
+					DAY_IN_SECONDS,
+					self::RUN_SCHEDULED_RECURRING_ACTIONS_HOOK,
+					[],
+					'ActionScheduler',
+					true,
+					20
+				);
+			}
+			wp_cache_set( 'as_is_ensure_recurring_actions_scheduled', true, HOUR_IN_SECONDS );
+		}
+	}
+
+	/**
+	 * Trigger the hook to allow other plugins to schedule their recurring actions.
+	 *
+	 * @return void
+	 */
+	public function run_recurring_scheduler_hook(): void {
+		/**
+		 * Fires to allow extensions to verify and ensure their recurring actions are scheduled.
+		 *
+		 * This action is scheduled to trigger once every 24 hrs for the purpose of having 3rd party plugins verify that
+		 * any previously scheduled recurring actions are still scheduled. Because recurring actions could stop getting
+		 * rescheduled by default due to excessive failures, database issues, or other interruptions, extensions can use
+		 * this hook to check for the existence of their recurring actions and reschedule them if necessary.
+		 *
+		 * Example usage:
+		 *
+		 * add_action('action_scheduler_ensure_recurring_actions', function() {
+		 *     // Check if the recurring action is scheduled, and reschedule if missing.
+		 *     if ( ! as_has_scheduled_action('my_recurring_action') ) {
+		 *         as_schedule_recurring_action( time(), HOUR_IN_SECONDS, 'my_recurring_action' );
+		 *     }
+		 * });
+		 *
+		 * @since 3.9.3
+		 */
+		do_action( 'action_scheduler_ensure_recurring_actions' );
+	}
+}