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/migration/DryRun_ActionMigrator.php

@@ -0,0 +1,28 @@
+<?php
+
+
+namespace Action_Scheduler\Migration;
+
+/**
+ * Class DryRun_ActionMigrator
+ *
+ * @package Action_Scheduler\Migration
+ *
+ * @since 3.0.0
+ *
+ * @codeCoverageIgnore
+ */
+class DryRun_ActionMigrator extends ActionMigrator {
+	/**
+	 * Simulate migrating an action.
+	 *
+	 * @param int $source_action_id Action ID.
+	 *
+	 * @return int
+	 */
+	public function migrate( $source_action_id ) {
+		do_action( 'action_scheduler/migrate_action_dry_run', $source_action_id ); // phpcs:ignore WordPress.NamingConventions.ValidHookName.UseUnderscores
+
+		return 0;
+	}
+}