DatePicker: improve accessibility semantics, correct SR announcements and keyboard focus behavior

[aligoknl]

This PR improves DatePicker accessibility semantics, screen reader announcement, and keyboard/focus behavior.

What changed

• Added/updated ARIA semantics for DatePicker triggers (dialog relationship and expanded state).
• Improved input-icon trigger accessibility parity.
• Refined keyboard handling for Enter to avoid unnecessary preventDefault.
• Improved calendar grid focus behavior (gridcell-focused navigation flow).
• Updated clear action behavior to:
  • clear model value,
  • close overlay,
  • restore focus to input safely without reopening the panel immediately.
• Correct screen reader announcements

Why

These changes improve screen reader and keyboard UX while preserving existing component behavior as much as possible.

Validation

• Ran DatePicker unit tests:

  • src/datepicker/DatePicker.spec.js passes.

• Ran a11y automated tool (used axe DevTools)

  • Before:

• After:

• Tested with screen reader (used Voiceover)

Notes

• Scope intentionally kept narrow to DatePicker behavior/accessibility.
• No intended functional changes outside DatePicker.

[martijnfleurkens]

Great PR! Since the critical accessibility issues currently in this datepicker primevue component it's breaking users which are using the keyboard and/or screen reader (or any other assistive device). Which makes it a component we currently can not use yet