🎨 Palette: Improve accessibility for scheduler task list icon buttons

[thirdeyenation]

💡 What: Added aria-label to icon-only buttons and aria-hidden="true" to their inner material-symbols-outlined spans in the scheduler task list.
🎯 Why: Screen readers would previously read the raw ligature text (e.g. 'play_arrow', 'delete') which is confusing for accessibility users.
📸 Before/After: N/A (Non-visual accessibility change)
♿ Accessibility: Enhances screen reader navigation by explicitly labeling icon buttons and hiding visual-only ligature spans.

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PR created automatically by Jules for task 11620020667846774494 started by @thirdeyenation

[google-labs-jules]

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