Update CommandBar to clip to bounds

[joannaquu]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

We apply a corner radius to CommandBar but don't set clipsToBounds, causing buttons to spill outside the container.

• Since we apply a shadow, we can't set it on the CommandBar itself so set it on the commandBarContainerStackView in it.

Verification

Visual Verification

Before	After

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