Add listbox-picker test case (custom dropdown ARIA listbox contract)

[manichandra]

Summary

Adds test_cases/listbox-picker/, a test case that evaluates a custom dropdown "picker" against the ARIA listbox contract — semantics that a static axe-core pass does not catch on a div-based dropdown.

Motivation

There is currently no listbox/combobox test case. Custom dropdown menus are a pattern models frequently generate without accessibility semantics:

• the trigger has no aria-haspopup / aria-expanded, so assistive technology cannot announce that it opens a popup or whether it is open;
• the popup is built from divs with no role="listbox" / role="option", so it is not exposed as a selectable list;
• options carry no aria-selected, so the current choice is not announced;
• there is no keyboard support (arrow keys, Escape).

axe-core does not flag a div dropdown that merely looks interactive, so these gaps go unmeasured. The case maps to WAI-ARIA 1.2, the WAI-ARIA APG listbox pattern, and WCAG 2.1 SC 4.1.2 (Name, Role, Value).

What this adds

• prompt.yaml — a neutral product spec for a text-style picker that explicitly forbids a native <select>. It does not state the ARIA answer, so the eval measures whether the model gets it right.
• test.js — six Requirement assertions, following the existing module.exports.run = async ({ page, assert, utils }) convention with { pass, message } results:
  1. Picker trigger exposes popup state (aria-haspopup and aria-expanded)
  2. Open menu uses role="listbox" with role="option" children
  3. Each option has an accessible name
  4. Options expose selection state with aria-selected (exactly one selected)
  5. Arrow keys move the active option
  6. Escape closes the listbox
• examples/ — two example files with embedded a11y-assertions expectations: a conformant ARIA listbox and a div-based dropdown anti-pattern.

DOM discovery is self-contained (it tags candidate options and inspects roles/attributes), so the case does not depend on widget-specific utils; the only utils call (reload) is guarded.

Validation

Ran test.js against both examples with node_runner:

• the conformant listbox passes all six assertions;
• the div-based dropdown fails the five role / selection / keyboard assertions (only "Each option has an accessible name" passes, since the items have text).

Checklist

• I have signed the Microsoft CLA
• I ran the repository's evaluation harness locally
• The new case follows the existing test_cases/ conventions