Align chat model picker ARIA semantics with single-selection menu behavior

[Copilot]

The chat model picker exposed checkbox semantics to assistive tech, which is misleading because model selection behaves as a single-choice selection list. This updates the picker to announce radio-style menu semantics while preserving selected-state announcements.

• Accessibility semantics

  • Switched action item role from menuitemcheckbox to menuitemradio in the model picker accessibility provider.
  • Kept selected-state wiring (isChecked) so the active model remains announced as selected within the menu.

• Code structure

  • Extracted picker accessibility mapping into getModelPickerAccessibilityProvider() in chatModelPicker.ts.
  • Reused the provider at widget show-time to keep role/state behavior centralized and testable.

• Coverage

  • Added a focused unit test validating:
    • action items => menuitemradio
    • separators => separator
    • widget role => menu

getRole: (element) => {
	switch (element.kind) {
		case ActionListItemKind.Action: return 'menuitemradio';
		case ActionListItemKind.Separator: return 'separator';
		default: return 'separator';
	}
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Model picker exposes checked checkbox state to screen readers but is not actionable</issue_title>
<issue_description>### Accessibility issue
The model picker exposes a checked/checkbox state to screen readers even though the control is not actionable as a checkbox.

Expected behavior

The model picker should expose a semantic role/state that matches its actual interaction model (for example, menuitemradio or option with selected state), and avoid announcing "checkbox" when users cannot toggle it as a checkbox.

Actual behavior

Screen readers announce checked checkbox semantics that are misleading and not actionable.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Model picker exposes checked checkbox state to screen readers but is not actionable #296609

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[Copilot]

Pull request overview

Updates the Chat Model Picker’s accessibility semantics so assistive technologies perceive it as a single-selection menu (radio-style) rather than a multi-toggle checkbox list, aligning ARIA roles with actual interaction behavior.

Changes:

• Switched model item ARIA role from menuitemcheckbox to menuitemradio while keeping checked-state announcements.
• Extracted the picker accessibility provider into getModelPickerAccessibilityProvider() and reused it at show-time.
• Added a unit test validating widget/item roles (menu, menuitemradio, separator).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/vs/workbench/contrib/chat/browser/widget/input/chatModelPicker.ts	Centralizes model picker list accessibility provider and updates action item role to menuitemradio.
src/vs/workbench/contrib/chat/test/browser/widget/input/chatModelPicker.test.ts	Adds coverage to ensure the accessibility provider returns the expected widget/item roles.