[iOS 26+] Improve glass support for BottomSheetController

[nodes11]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

This PR brings in a few changes to improve the liquid glass support for the BottomSheetController.

• Add a usesAdaptiveBackground public property which clients can set so that the sheet background changes from glass to solid when transitioning from a partial sheet to a full sheet. This matches the behavior seen with the native sheets.
• Add helper methods aid the transition from a glassy background to a solid background
• Don't apply fluent shadow to bottomsheetview when glass is being used and the OS >= 26. Without this change, the header UINavigationBar renders incorrectly.

Binary change

(how is our binary size impacted -- see https://github.com/microsoft/fluentui-apple/wiki/Size-Comparison)

Verification

• Validated using internal use cases
• Validated in the demo controller

Visual Verification

Simulator.Screen.Recording.-.iPhone.17.Pro.-.2026-07-13.at.13.40.51.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