Add placeholder MediaContentAccessCheck class to fix error when installing

[ruscoe]

Line 50 of dropai.services.yml names a class that does not exist (Drupal\dropai\Access\MediaContentAccessCheck). This causes the following error when trying to install the DropAI module on Drupal 10.3.1:

Uncaught PHP Exception TypeError: "in_array(): Argument #2 ($haystack) must be of type array, bool given" at /var/www/d10/core/lib/Drupal/Core/DependencyInjection/Compiler/RegisterAccessChecksPass.php line 46

This pull request adds a placeholder class that resolves the error.