[macOS] Fix ObjC export of MultilinePillPickerView

[mischreiber]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

My previous PR #2121 didn't work in ObjC as expected. Since we don't have a real ObjC demo in macOS yet, this was manually validated to quickly unblock the issue.

(The problem was inheriting from a generic class -- that silently fails to export to Objective-C, even though there's an @objc in the title.)

Binary change

n/a, still can't measure macOS

Verification

Verified MPP component still renders as expected.

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