[FluentNotification] Add Expand button behavior

[nodes11]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

This change introduces an optional expand button for FluentNotification when there is a message line limit set and the message text is longer than the given limit. Tapping the expand button will cause all text to be displayed. This is achieved through the addition of a new SwiftUI view called ExpandableText. Consumers can enable this feature by setting enableExpandableMessageText to true on the FluentNotification or the corresponding state object (depending on the initialization method).

Binary change

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

Verification

Validated in the fluent demo controller, as well as in internal use cases.

Visual Verification

ExpandableText Demo:

Simulator.Screen.Recording.-.iPhone.17.Pro.-.2026-03-31.at.16.11.53.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