Issue #3579956: Improve test coverage with widget UI interaction, submission verification, and facets JS fix

Changes:

• All FunctionalJavascript tests now interact with the actual select-a11y widget UI (button click, search input, suggestion click) instead of setting values directly on the hidden native select.
• Search functionality is tested: the single select test types in the search input, verifies filtering reduces suggestions to the matching one, then clicks it. The search disabled test verifies the search input has the hidden attribute.
• Submission verification added to all submodule tests: BEF filter selects "a" via search, submits and verifies "Page One" shown / "Page Two" filtered out. Webform selects "Banana", submits and verifies the WebformSubmission data. Facets clicks a suggestion, verifies page navigation and filtered result count.
• Facets JS bug fix: select-a11y-ng-facets.js used e.detail on the change event which is always undefined — replaced with element.selectedIndex so facet option selection actually triggers navigation.

Closes #3579956