Add filters with order_id for custom receipt generation

This PR adds two filters to allow developers to customize receipt generation:

1. `robokassa_receipt_pre` - Called at the start of createRobokassaReceipt().
   Return non-null value to bypass default receipt creation.

2. `wc_robokassa_receipt` - Now passes $order_id as second argument.
   Allows modifying the generated receipt with access to order data.

Both filters pass $order_id, enabling:
- Custom pricing calculations (discounts, dynamic prices)
- Integration with other plugins without core modifications
- Access to order data for receipt customization

Usage examples:

// Pre-filter: complete override
add_filter('robokassa_receipt_pre', function($receipt, $order_id) {
    $order = wc_get_order($order_id);
    // Custom receipt generation logic
    return $receipt;
}, 10, 2);

// Post-filter: modify existing receipt
add_filter('wc_robokassa_receipt', function($receipt, $order_id) {
    $order = wc_get_order($order_id);
    // Modify receipt items, totals, etc.
    return $receipt;
}, 10, 2);

Backward compatible: existing filters without order_id argument will continue to work.

Author: Aleksei Tikhomirov

wp_robokassa.php

@@ -618,6 +618,21 @@ function getRobokassaPasses()
  */
 function createRobokassaReceipt($order_id)
 {
+	/**
+	 * Filter to allow custom receipt generation.
+	 * Return a non-null value to bypass default receipt creation.
+	 *
+	 * @param array|null $pre_receipt Custom receipt data or null to use default.
+	 * @param int        $order_id    Order ID.
+	 *
+	 * @since 1.6.6
+	 */
+	$pre_receipt = apply_filters('robokassa_receipt_pre', null, $order_id);
+
+	if ($pre_receipt !== null) {
+		return $pre_receipt;
+	}
+
 	global $woocommerce;
 	$order = new WC_Order($order_id);
 
@@ -722,7 +737,7 @@ function createRobokassaReceipt($order_id)
 		robokassa_payment_DEBUG('Robokassa: общая сумма чека (' . $total_receipt . ') НЕ совпадает с общей суммой заказа (' . $total_order . ')');
 	}
 
-	return apply_filters('wc_robokassa_receipt', $receipt);
+	return apply_filters('wc_robokassa_receipt', $receipt, $order_id);
 }
 
 /**