Refactor `siAutocomplete` to use `@angular/aria/combobox` and `@angular/aria/listbox`

[chintankavathia]

Background

siAutocomplete currently owns combobox/listbox relationships, active-option management, and
keyboard behavior that can be provided by Angular ARIA. The evaluation POC applies the
combobox and listbox primitives to the shared autocomplete implementation and its consumers,
including typeahead, si-select, and si-custom-select.

This migration has a wider impact than the other aria related refactoring tasks because changing the shared directives also affects components that compose them.

Related to #1070.

Implementation reference:
angular/aria/evaluation

Scope

• Adopt @angular/aria/combobox in siAutocomplete and
  @angular/aria/listbox for its popup options.
• Productionize the POC across all direct internal consumers, including typeahead, si-select,
  filtered si-select, and si-custom-select.
• Audit other siAutocomplete consumers and update them in the same task.
• Remove duplicate active-descendant, option-ID, and keyboard-manager logic that Angular ARIA
  now owns.