Autocomplete elements

Problem/Motivation

Part 9 of splitting #3595219: 3.0 Exploration. This issue adds the two autocomplete element types:

• webform_autocomplete — a text field with a configured list of autocomplete suggestions.
• entity_autocomplete — a reference field that autocompletes against entities, including its default referenced entities.

Neither is exposed in 3.x today.

Proposed resolution

• webform_autocomplete: expose its fields and resolve autocompleteItems as a plain array of strings, backed by a WebformAutocompleteMatchOperator enum.
• entity_autocomplete: add a webform_entity_autocomplete producer exposing the element's configuration, and a webform_element_entity_autocomplete_default_entities producer that resolves the default referenced entities (with their cacheability — building on the default-value/cacheability foundation in #3598935: Improve cacheability and rendering).

Remaining tasks

• Add src/Enum/WebformAutocompleteMatchOperator.php; wire into WebformSchemaBuilder.php.
• Add producers WebformEntityAutocomplete.php and WebformElementEntityAutocompleteDefaultEntities.php; resolve autocompleteItems as [String].
• Register both element types in WebformExtension.php.
• Tests: AutocompleteTest, EntityAutocompleteDefaultEntitiesTest, WebformEntityAutocompleteTest, plus the autocomplete.gql / webform_entity_autocomplete.gql fixtures and shared test-form additions.

API changes

Additive: new webform_autocomplete and entity_autocomplete element types, the WebformAutocompleteMatchOperator enum, and default-entity resolution for entity autocomplete. No fields removed.

Release notes snippet

The webform-autocomplete and entity-autocomplete element types are now supported, including autocomplete suggestion lists and default referenced entities.