Workspace AJAX query parameters become arrays when form elements have pre-existing token/persist values

Problem/Motivation

When FormOperations::setAjaxWorkspace() processes a form element that already has token or persist in its #ajax['options']['query'] (but no workspace key), the use of array_merge_recursive() converts these scalar values into arrays. This causes Symfony's InputBag::get() to throw a BadRequestException: "Input value contains a non-scalar value."

Steps to reproduce

This can occur when modules like Media Library copy request parameters to form elements, and those parameters already contain workspace-related values from a previous AJAX request. Hopefully (or hopefully not?), others run into this and help with the steps to reproduce because I'm not totally sure how to reproduce this on vanilla Drupal core because my use case has a lot of stuff going on, but it is something like

• Enable Workspaces module
• Create and switch to a non-default workspace
• Build a workspace-safe form with an AJAX element that has pre-existing token or persist query params (simulating what happens when Media Library copies request params)
• The resulting #ajax['options']['query'] will contain arrays instead of scalars:
• token becomes ['workspace-token', 'pre-existing-token']
• persist becomes [false, '']

Proposed resolution

Replace array_merge_recursive() with array_replace() in FormOperations::setAjaxWorkspace(). This ensures workspace parameters always remain scalar values, with the current workspace's values taking precedence over any pre-existing values.

<?php
  // Before
  if (isset($element['#ajax']) && !isset($element['#ajax']['options']['query']['workspace'])) {
    $element['#ajax']['options']['query'] = array_merge_recursive(
      $url_query_options,
      $element['#ajax']['options']['query'] ?? [],
    );
  }

  // After
  if (isset($element['#ajax'])) {
    $existing_query = $element['#ajax']['options']['query'] ?? [];
    $element['#ajax']['options']['query'] = NestedArray::mergeDeep($existing_query, $url_query_options);
  }
?>