Fix PHP 8.2+ deprecation warnings for dynamic property creation (#95)

Declare protected properties in WooViet_OnePay_Abstract class to resolve
deprecation warnings when creating dynamic properties in PHP 8.2+.

Properties declared:
- $testmode
- $merchant_id
- $access_code
- $secure_secret
- $user
- $password
- $debug

This fix is backward compatible with older PHP versions and resolves
the warnings reported in issue #94.

Fixes #94

Co-authored-by: Claude <noreply@anthropic.com>

inc/onepay/abstract-payment.php

@@ -18,6 +18,27 @@ abstract class WooViet_OnePay_Abstract extends WC_Payment_Gateway {
 	/** @var WC_Logger Logger instance */
 	public static $log = false;
 
+	/** @var bool Test mode flag */
+	protected $testmode;
+
+	/** @var string OnePay merchant ID */
+	protected $merchant_id;
+
+	/** @var string OnePay access code */
+	protected $access_code;
+
+	/** @var string OnePay secure secret */
+	protected $secure_secret;
+
+	/** @var string OnePay user */
+	protected $user;
+
+	/** @var string OnePay password */
+	protected $password;
+
+	/** @var bool Debug mode flag */
+	protected $debug;
+
 	/**
 	 * Configure $method_title and $method_description
 	 */