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_FatalErrorMonitor.php

@@ -0,0 +1,98 @@
+<?php
+
+/**
+ * Class ActionScheduler_FatalErrorMonitor
+ */
+class ActionScheduler_FatalErrorMonitor {
+
+	/**
+	 * ActionScheduler_ActionClaim instance.
+	 *
+	 * @var ActionScheduler_ActionClaim
+	 */
+	private $claim = null;
+
+	/**
+	 * ActionScheduler_Store instance.
+	 *
+	 * @var ActionScheduler_Store
+	 */
+	private $store = null;
+
+	/**
+	 * Current action's ID.
+	 *
+	 * @var int
+	 */
+	private $action_id = 0;
+
+	/**
+	 * Construct.
+	 *
+	 * @param ActionScheduler_Store $store Action store.
+	 */
+	public function __construct( ActionScheduler_Store $store ) {
+		$this->store = $store;
+	}
+
+	/**
+	 * Start monitoring.
+	 *
+	 * @param ActionScheduler_ActionClaim $claim Claimed actions.
+	 */
+	public function attach( ActionScheduler_ActionClaim $claim ) {
+		$this->claim = $claim;
+		add_action( 'shutdown', array( $this, 'handle_unexpected_shutdown' ) );
+		add_action( 'action_scheduler_before_execute', array( $this, 'track_current_action' ), 0, 1 );
+		add_action( 'action_scheduler_after_execute', array( $this, 'untrack_action' ), 0, 0 );
+		add_action( 'action_scheduler_execution_ignored', array( $this, 'untrack_action' ), 0, 0 );
+		add_action( 'action_scheduler_failed_execution', array( $this, 'untrack_action' ), 0, 0 );
+	}
+
+	/**
+	 * Stop monitoring.
+	 */
+	public function detach() {
+		$this->claim = null;
+		$this->untrack_action();
+		remove_action( 'shutdown', array( $this, 'handle_unexpected_shutdown' ) );
+		remove_action( 'action_scheduler_before_execute', array( $this, 'track_current_action' ), 0 );
+		remove_action( 'action_scheduler_after_execute', array( $this, 'untrack_action' ), 0 );
+		remove_action( 'action_scheduler_execution_ignored', array( $this, 'untrack_action' ), 0 );
+		remove_action( 'action_scheduler_failed_execution', array( $this, 'untrack_action' ), 0 );
+	}
+
+	/**
+	 * Track specified action.
+	 *
+	 * @param int $action_id Action ID to track.
+	 */
+	public function track_current_action( $action_id ) {
+		$this->action_id = $action_id;
+	}
+
+	/**
+	 * Un-track action.
+	 */
+	public function untrack_action() {
+		$this->action_id = 0;
+	}
+
+	/**
+	 * Handle unexpected shutdown.
+	 */
+	public function handle_unexpected_shutdown() {
+		$error = error_get_last();
+
+		if ( $error ) {
+			if ( in_array( $error['type'], array( E_ERROR, E_PARSE, E_COMPILE_ERROR, E_USER_ERROR, E_RECOVERABLE_ERROR ), true ) ) {
+				if ( ! empty( $this->action_id ) ) {
+					$this->store->mark_failure( $this->action_id );
+					do_action( 'action_scheduler_unexpected_shutdown', $this->action_id, $error );
+				}
+			}
+
+			$this->store->release_claim( $this->claim );
+		}
+	}
+}