feat: Add ARIA combobox pattern to autocomplete search components

[akj]

This implements the WAI-ARIA combobox pattern on the three AllDataSearch / Autocomplete inputs in the popup (Users, Objects, Shortcuts), so screen readers can announce the dropdown, its options, and which option is currently active.

This is the autocomplete portion of #1109, split out as I offered there. The link-to-button (WCAG 4.1.2) conversion from that PR will follow as a separate PR for #1107, so each change can be reviewed on its own. Addresses #1108.

Changes

• role=combobox on each input, with aria-expanded, aria-autocomplete=list, aria-haspopup=listbox, aria-controls, and aria-activedescendant
• role=listbox plus a unique id on each dropdown; role=option, a unique id, and aria-selected on each item
• an onAriaStateChange callback so the parent input (which owns the element) mirrors the Autocomplete child's open / active-index state, instead of lifting that state up
• a unique idPrefix per instance so the ARIA ids don't collide across the three widgets
• a visible .slds-is-selected highlight for the active option, so sighted keyboard users get the same cue screen readers do (SLDS only ships this styling for buttons)
• three virtual-scroll fixes (item height measurement, scroll-into-view math, and keeping the selected item inside the rendered range) so aria-activedescendant always points at an element that is actually in the DOM

Testing

• Tested manually with NVDA.
• Added an e2e test ("Autocomplete Accessibility (ARIA combobox)" in popup.spec.js) that runs in the existing mock harness, so it needs no org. It checks the role wiring, aria-expanded toggling on open and on Escape, the listbox/option markup, and that aria-activedescendant resolves to a rendered, aria-selected option.

As with the previous PR, I'm a screen reader user and a little outside my lane on the visual side, so please flag anything that looks off visually. Thanks!