feat(ui5-list): add live region announcement for item selection (POC)

[NakataCode]

Problem:

• When a user selects or deselects a list item with a mouse click, screen readers get no immediate feedback. They only discover the selection state when navigating back to the item, because the AT reads aria-selected on focus.

Solution:

• Call announce() (InvisibleMessage) with a polite live region announcement immediately after a selection change succeeds. The text is "Selected" or "Not Selected" using the existing i18n strings LIST_ITEM_SELECTED / LIST_ITEM_NOT_SELECTED.

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What's covered:

• All selectable modes: Single, SingleStart, SingleEnd, SingleAuto, Multiple
• Mouse clicks and keyboard (Space/Enter)
• SingleAuto arrow-key navigation (goes through the same onSelectionRequested path)
• No announcement when selectionMode="None"
• No announcement when the consumer prevents the selection-change event
• Delete mode is naturally excluded — it never toggles selected

[ui5-webcomponents-bot]

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