Layout commandBarContainerStackView in CommandBar

[joannaquu]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

In certain edge cases, if commandBarContainerStackView hasn't been laid out, its stale values will be used when calculating the scroll view shadow. (if containerView.frame.width is 0). This will cause the CAGradientLayer to hide the entire command bar, not just the scroll edge.

Make sure we layout commandBarContainerStackView.

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)

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