Multi-value field widget table renders an empty <th> for the actions (Remove) column

Problem/Motivation

Since Drupal 10.2 (#1038316), the table rendered for multi-value field widgets has a column for the per-item "Remove" button. The header cell for that column is an empty array, so it renders as an empty <th></th>.

In 11.x this is in \Drupal\Core\Field\FieldPreprocess::preprocessFieldMultipleValueForm() (previously template_preprocess_field_multiple_value_form()):

$header = [
  [
    'data' => [ /* h4 field label */ ],
    'colspan' => 2,
    'class' => ['field-label'],
  ],
  [],
  $this->t('Order', [], ['context' => 'Sort order']),
];

Accessibility checkers report an empty table header on every form with an unlimited/multi-value field: axe-core's empty-table-header rule and Editoria11y's "Empty table header" check. Screen reader users get no name for the column that holds the Remove buttons. This relates to WCAG 1.3.1 Info and Relationships.

#3587682 reported an empty table header from an automated scan, but it was closed as "cannot reproduce". That report pointed at a different page. This issue is the reproducible case in the field widget table.

Steps to reproduce

1. Install Drupal with the Standard profile (Claro as the admin theme).
2. Add a reference field (paragraph reference) with unlimited cardinality to the Article content type.
3. Go to /node/add/article.
4. Inspect the field's table header: the third <th> is empty.
5. Run axe DevTools or Editoria11y on the page: it reports an empty table header.

Proposed resolution

Give the actions column header a visually hidden label so it has an accessible name without changing the visual design:

[
  'data' => [
    '#type' => 'html_tag',
    '#tag' => 'span',
    '#value' => $this->t('Operations'),
    '#attributes' => ['class' => ['visually-hidden']],
  ],
],

"Operations" matches the label core already uses for action columns elsewhere, such as entity list builders. Claro's preprocess_field_multiple_value_form keeps working: for disabled widgets, its loop adds the is-disabled class to header cells that have a data key.

Alternative: only output the column (header and cells) when at least one item has _actions. That also fixes the empty body cells contrib widgets get (see Paragraphs #3418303). It is a larger change and could be a follow-up.

Remaining tasks

• Review the approach and the label wording.
• Add test coverage that the header cell is not empty, e.g. in an existing multi-value widget functional test.
• Convert to an MR.

User interface changes

No visual change. Screen readers announce the actions column header as "Operations".

API changes

None.

Data model changes

None.

Release notes snippet

N/A