[FluentList, ListItem, BottomSheetController] Updates for glass support

[nodes11]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

• Bottom sheets: iOS 26 glass sheets now use the sheet’s background token as a tint—65% opacity for glass states and fully opaque when expanded with adaptive backgrounds. This improves theme consistency and aligns with Apple's system sheets. Older iOS and visionOS are unaffected.
• FluentList: Adds the public .glass list style. It retains the inset-grouped layout while removing list and item backgrounds, allowing underlying glass/material content to remain visible.
• ListItem: Automatically use clear backgrounds under .glass. Custom background styles still take precedence.
• BottomSheetControllerDemo - Add a UIMenu table view cell that allows the user to select between glass and primary sheet styles. Add a toggle to enable the adaptive background mode when glass is enabled.

Binary change

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

Verification

Validated with internal test cases.

Simulator.Screen.Recording.-.iPhone.17.Pro.-.2026-08-03.at.07.25.03.mov

Simulator.Screen.Recording.-.iPhone.17.Pro.-.2026-08-03.at.07.28.33.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