fix(react-aria): only announce combobox section changes to VoiceOver

[gonzoblasco]

Summary

Fixes #10220: ComboBox only announced the item name with VoiceOver, losing the richer native announcement (selected state, menu item role, item count).

Root cause

On Apple devices, useComboBox announces every focused-item change through a live region. Current VoiceOver versions announce per-item details (selected state, role, item count) natively while arrowing within a section, so that live-region announcement interrupts and replaces the richer native speech with just the item name.

Change

Gate the manual announcement so it only fires when navigating into a new section, where VoiceOver is silent and the section title + option count are not announced natively. Arrowing within the same section no longer triggers a manual announcement, letting VoiceOver handle it.

This keeps the inter-section navigation announcements (section title + option count) that were the reason the effect existed, while no longer stomping on VoiceOver's native per-item announcements.

Tests

• Updated the combobox announcement tests to assert that per-item navigation no longer triggers a manual announcement, while section-entry announcements are preserved.
• 218 tests pass in packages/@adobe/react-spectrum/test/combobox/ComboBox.test.js.
• 41 tests pass in packages/react-aria-components/test/ComboBox.test.js.

Tested scenario matrix under mocked Apple platform:

• Arrowing through items (no sections): no manual announcement.
• Arrowing to the selected item: no manual announcement.
• Entering a section: announces "Entered group X, with N options. Item".
• Moving within a section: no manual announcement.
• Selection: still announces "Item, selected".