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_InvalidActionException.php

@@ -0,0 +1,47 @@
+<?php
+
+/**
+ * InvalidAction Exception.
+ *
+ * Used for identifying actions that are invalid in some way.
+ *
+ * @package ActionScheduler
+ */
+class ActionScheduler_InvalidActionException extends \InvalidArgumentException implements ActionScheduler_Exception {
+
+	/**
+	 * Create a new exception when the action's schedule cannot be fetched.
+	 *
+	 * @param string $action_id The action ID with bad args.
+	 * @param mixed  $schedule  Passed schedule.
+	 * @return static
+	 */
+	public static function from_schedule( $action_id, $schedule ) {
+		$message = sprintf(
+			/* translators: 1: action ID 2: schedule */
+			__( 'Action [%1$s] has an invalid schedule: %2$s', 'action-scheduler' ),
+			$action_id,
+			var_export( $schedule, true ) // phpcs:ignore WordPress.PHP.DevelopmentFunctions.error_log_var_export
+		);
+
+		return new static( $message );
+	}
+
+	/**
+	 * Create a new exception when the action's args cannot be decoded to an array.
+	 *
+	 * @param string $action_id The action ID with bad args.
+	 * @param mixed  $args      Passed arguments.
+	 * @return static
+	 */
+	public static function from_decoding_args( $action_id, $args = array() ) {
+		$message = sprintf(
+			/* translators: 1: action ID 2: arguments */
+			__( 'Action [%1$s] has invalid arguments. It cannot be JSON decoded to an array. $args = %2$s', 'action-scheduler' ),
+			$action_id,
+			var_export( $args, true ) // phpcs:ignore WordPress.PHP.DevelopmentFunctions.error_log_var_export
+		);
+
+		return new static( $message );
+	}
+}