🎨 Palette: [Accessibility] Add proper ARIA attributes to chat navigation icon-only buttons

[thirdeyenation]

💡 What: Added aria-label to the chat navigation buttons and aria-hidden="true" to their inner span icons.
🎯 Why: Prevents screen readers from confusingly reading the raw Material Symbol ligature text (e.g., 'vertical_align_top') instead of its actual purpose.
📸 Before/After: Visual presentation remains completely unaffected.
♿ Accessibility: Ensures that screen reader users are provided with clear, semantic names for navigation actions.

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PR created automatically by Jules for task 11450767128234003666 started by @thirdeyenation

[google-labs-jules]

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