#3588342: Alt text rules not enforced when using Media Library

Skip AJAX validation on all entity types except media. This preserves the fix from #3541613 (prevent spurious validation on widget AJAX like paragraphs add) while allowing media entities to validate over AJAX for Media Library saves.

Changes:

• Modified isAjaxRequest() to check entity type ID
• Media entities now validate over AJAX
• Non-media entities continue to skip AJAX validation
• Uses AjaxHelperTrait::isAjax() to check for AJAX, which should cover any standard Drupal AJAX requests
• Fixes Media Library widget validation bypass
• Maintains backward compatibility with #3541613 fix

Manual Testing

• Media Library AJAX save (the fix): Alt text validation errors now appear
• Direct media edit (/media/add/image, /media/{id}/edit): Still validates correctly
• Node form widget AJAX (paragraphs, field add): No spurious validation (preserves #3541613)

Closes #3588342 Related to #3541613