[Issue] Fixed 'Undefined class constant' error when interceptor is generated.

[m2-assistant]

This issue is automatically created based on existing pull request: #28797: Fixed 'Undefined class constant' error when interceptor is generated.

---

Summary (*)

1. Magento 2.4-develop
2. Since constant is declared as private it's not accessible from parent classes.
3. Since it used with static keyword, PHP will search it within class instance.
4. Class instance will be an instance of generated Interceptor without access to private variables of child classes.
5. This will lead to an 'Undefined class constant' error.

Examples (*)

1. Create a plugin for Magento\Theme\Block\Html\Title class:

   <type name="Magento\Theme\Block\Html\Title">
       <plugin name="titlePlugin" type="Magento\Theme\Plugin\TitlePlugin" />
   </type>

   <?php
   
   namespace Magento\Theme\Plugin;
   
   class TitlePlugin
   {
       /**
        * @param mixed $subject
        * @param mixed $result
        * @return mixed
        */
       public function afterGetPageHeading(
           $subject,
           $result
       ) {
           return $result;
       }
   }

2. Open frontend.

3. You'll see an error:

   Error: Undefined class constant 'XML_PATH_HEADER_TRANSLATE_TITLE' in app/code/Magento/Theme/Block/Html/Title.php:104 
   Stack trace: 
   #0 app/code/Magento/Theme/Block/Html/Title.php(82): Magento\Theme\Block\Html\Title->shouldTranslateTitle() 
   #1 generated/code/Magento/Theme/Block/Html/Title/Interceptor.php(32): Magento\Theme\Block\Html\Title->getPageHeading() 
   #2 app/code/Magento/Theme/view/frontend/templates/html/title.phtml(12): Magento\Theme\Block\Html\Title\Interceptor->getPageHeading()
   ...
   

Proposed solution

Another way to solve this bug - is to declare constant as public instead of private.
If you think it should be public - I'll update PR.

[m2-assistant]

Hi @engcom-Delta. Thank you for working on this issue.
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