[macOS] Announce DatePickerController changes in VoiceOver

[mischreiber]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

• Create a new NSAccessibilityStepper for the header, making it possible to read and change the value of the header as one entire element.
• Add an NSAccessibility announcement when the visible month in the DatePickerController changes.
• Clean up TestDatePickerController so popover-hosted picker works as expected.

Binary change

n/a, no Mac size testing

Verification

• Verified VoiceOver announcements happen for all three date pickers in the test screen.
• Ensured announcements happen exactly once, and only when the picker changes (not on initial appearance).

Stepper.mov

Pull request checklist

This PR has considered:

• Light and Dark appearances
• iOS supported versions (all major versions greater than or equal current target deployment version)
• VoiceOver and Keyboard Accessibility
• Internationalization and Right to Left layouts
• Different resolutions (1x, 2x, 3x)
• Size classes and window sizes (iPhone vs iPad, notched devices, multitasking, different window sizes, etc)
• iPad Pointer interaction
• SwiftUI consumption (validation or new demo scenarios needed)
• Objective-C exposure (provide it only if needed)

Microsoft Reviewers: Open in CodeFlow