[Autocomplete] Translate the optgroup labels returned by the AJAX endpoint

[kira0269]

Q	A
Bug fix?	yes
New feature?	no
Issues	-
License	MIT

The group_by option of an autocomplete field produces the <optgroup> labels. When the <select> is rendered by the form theme, those labels go through |trans:

<optgroup label="{{ choice_translation_domain is same as(false) ? group_label : group_label|trans({}, choice_translation_domain) }}">

But the labels sent as JSON by the autocomplete endpoint are emitted verbatim by AutocompleteResultsExecutor:

$optgroups = array_map(static fn (string $label) => ['value' => $label, 'label' => $label], array_unique($optgroupLabels));

For an AJAX autocompleter this is the path that matters: the server-rendered <select> only holds the preselected option(s), so every group the user sees while typing comes from that JSON. Using a translation key in group_by therefore shows the raw key. It also makes the labels disagree with the server-rendered ones, which the Stimulus controller looks up by label:

const optgroupElement = parentElement.querySelector(`optgroup[label="${optgroupData.label}"]`);

What this PR does

• AutocompleteResultsExecutor takes an optional TranslatorInterface and translates the optgroup labels. Values returned by group_by that implement TranslatableInterface are no longer cast to string either (casting a TranslatableMessage is a fatal error).
• The domain comes from a new optional getTranslationDomain(): string|false|null method on the autocompleter — null uses the default domain, false disables the translation. It is declared as a @method on EntityAutocompleterInterface and commented out in the interface body, like getGroupBy(), and detected with method_exists(), so existing autocompleters keep working untouched.
• WrappedEntityTypeAutocompleter implements it by reading the field's choice_translation_domain option, which is exactly what the form theme uses. So the AJAX results and the rendered <select> now agree.
• The optgroup value keeps the untranslated label, so it still ties a result to its group whatever the locale is. Only the label is translated.

BC

The translator is an optional constructor argument wired with NULL_ON_INVALID_REFERENCE, and without it the previous behavior is kept. Autocompleters that do not implement getTranslationDomain() translate in the default domain, which matches what the form theme already does for the same labels.