Fixed 'Undefined class constant' error when interceptor is generated.

[vovayatsyuk]

Description (*)

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

Manual testing scenarios (*)

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()
   ...
   

Questions or comments

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.

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds are green)

Resolved issues:

1. resolves [Issue] Fixed 'Undefined class constant' error when interceptor is generated. #28981: Fixed 'Undefined class constant' error when interceptor is generated.

[m2-assistant]

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[ihor-sviziev]

@magento run all tests

[magento-engcom-team]

Hi @ihor-sviziev, thank you for the review.
ENGCOM-7713 has been created to process this Pull Request

[engcom-Delta]

✔️ QA passed
Was checked case from Manual testing scenario
Before:
❌ Error "Undefined class constant 'XML_PATH_HEADER_TRANSLATE_TITLE' " is shown

After:
✔️ No errors when plugin is loaded

[engcom-Kilo]

@magento run all tests

[engcom-Delta]

@magento create issue

[slavvka]

@magento run WebAPI tests

[slavvka]

@magento run WebAPI Tests

[slavvka]

@magento run Integration Tests

[m2-assistant]

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