[Bottom Sheet] Merge duplicate detents + other fixes

[lukas-cap]

1. We saw a bug where the partial detent height wasn't accounting for bottom safe area.

2. While fixing that, I noticed we have a lot of bugs around detent availability in restricted height scenarios. Dynamic height resolvers can produce a partial height effectively equal to collapsed or expanded. This caused invisible state cycling on handle taps, incorrect pan snapping, and meaningless delegate notifications.

Add supportsCollapsedState and supportsPartialState computed properties that gate detent availability. Partial is skipped when its height is within a merge threshold of collapsed or expanded. Collapsed is skipped when the sheet is height-restricted. Layout changes that invalidate the current detent reposition the sheet to expanded with delegate notification.

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

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Binary change

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

Verification

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Visual Verification

Before	After
Screenshot or description before this change	Screenshot or description with this change

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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