New #ajax['suppress_required_fields_validation'] option to skip required-field validation during AJAX submissions

Form elements with an #ajax callback can now opt out of required-field validation during the AJAX submission, without disabling other validation. Previously the only option was #limit_validation_errors = [], which suppresses all validation, including custom validators.

$form['my_button']['#ajax'] = [
    'callback' => '::myCallback',
    'suppress_required_fields_validation' => TRUE,
  ];

When set, required-field errors are skipped while the AJAX request rebuilds the form; all other validation still runs. The opt-in is read from the server-side form structure (the triggering element's #ajax settings), so it cannot be triggered by a crafted request. Required validation still applies on the final, non-AJAX form submission.