[material_ui] Assign SemanticsRole.option to Autocomplete option items #194031

Autocomplete's option items never get SemanticsRole.option, so on web the options popup is exposed as button children (with aria-current for the highlighted one) rather than option children with aria-selected.

#192971 adds SemanticsRole.listBox / SemanticsRole.option to the framework and engine. It cannot assign option to the items: RawAutocomplete only sees the opaque widget returned by the caller's optionsViewBuilder, and the default item builder lives in material_ui (flutter/packages), which is tested against stable and can't reference the new enum yet.

Follow-up (this issue), in flutter/packages/packages/material_ui:

• lib/src/autocomplete.dart _AutocompleteOptions: wrap the options view in Semantics(role: SemanticsRole.listBox, container: true, explicitChildNodes: true).
• lib/src/autocomplete.dart _AutocompleteOptionsList: add role: SemanticsRole.option to the per-item Semantics(button: true, selected: highlight, ...) wrapper (selected: landed in [material_ui] Improve Autocomplete, DrawerHeader, and Tooltip accessibility packages#12918). The web engine resolves the explicit role before the button flag, and the framework's option check only requires a listBox ancestor, so the ListView scroll wrappers between them are fine.
• lib/src/dropdown_menu.dart: entries are MenuItemButtons inside a MenuAnchor with no listBox container, and DropdownMenu is a combobox pattern, so this needs a design decision (listbox vs. menu popup) rather than a one-liner. Evaluate separately.
• Re-verify #/auto-complete in packages/material_ui/test_apps/a11y_assessments (Chrome Accessibility.getFullAXTree should show role=listbox > role=option with aria-selected).

Blocked on #192971 shipping in a stable Flutter release; material_ui then needs a min-Flutter bump to that version (cf. #192839 for why it can't reference master-only APIs under the current >=3.47.0 constraint).

Related VPAT: b/464138739, b/460080184. Related: #156371.