Set access modifier for $error_message to public and added a setter method (#46642)

* Set access modifier for $error_message to public and added a setter method.

* Add changefile(s) from automation for the following project(s): woocommerce

---------

Co-authored-by: github-actions <github-actions@github.com>

plugins/woocommerce/changelog/46642-fix-set_property_access_modifier_to_public

@@ -0,0 +1,4 @@
+Significance: patch
+Type: fix
+
+Fixed a bug causing incompatibility with 3rd-party coupon extensions when certain conditions were met.

plugins/woocommerce/includes/class-wc-coupon.php

@@ -84,9 +84,14 @@ class WC_Coupon extends WC_Legacy_Coupon {
 	/**
 	 * Error message.
 	 *
+	 * This property should not be considered public API, and should not be accessed directly.
+	 * It is being added to supress PHP > 8.0 warnings against dynamic property creation, and all access
+	 * should be through the getter and setter methods, namely `get_error_message()` and `set_error_message()`.
+	 * In the future, the access modifier may be changed back to protected.
+	 *
 	 * @var string
 	 */
-	protected $error_message;
+	public $error_message;
 
 	/**
 	 * Sorting.
@@ -871,6 +876,17 @@ class WC_Coupon extends WC_Legacy_Coupon {
 		return $this->error_message;
 	}
 
+	/**
+	 * Sets the error_message string.
+	 *
+	 * @param string $message Message string.
+	 *
+	 * @return void
+	 */
+	public function set_error_message( string $message ) {
+		$this->error_message = $message;
+	}
+
 	/**
 	 * Check if a coupon is valid for the cart.
 	 *