AC-6731: Validation fixes for webAPI

Author: sreenia806

app/code/Magento/Quote/Model/BillingAddressManagement.php

@@ -3,6 +3,7 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+declare(strict_types=1);
 
 namespace Magento\Quote\Model;
 

app/code/Magento/Quote/Model/QuoteAddressValidator.php

@@ -3,6 +3,8 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+declare(strict_types=1);
+
 namespace Magento\Quote\Model;
 
 use Magento\Framework\Exception\NoSuchEntityException;