Fix sheets ignoring their detents

[colemancda]

A full-size .sheet came up as a short strip at the bottom of the screen instead of covering it.

Cause

The Swift side has emitted a detents prop since sheets landed, but the interpreter never read it. Every sheet was therefore the same default ModalBottomSheet, and a Material sheet is only as tall as its content — so the sheet wrapped its two small views and stopped there. The same omission made .presentationDetents([.medium]) a no-op: both sheets in the gallery rendered identically.

Fix

Read the detents and size the sheet accordingly:

• no detents (SwiftUI's default) or .large → fills the height, and skips the partially-expanded stop since there's no partial detent to rest at
• .medium only → rests at half height
• both → starts partial, expands to full

The height has to be applied to the sheet's content (fillMaxHeight), not just the sheet state: skipPartiallyExpanded only chooses which anchor the sheet settles on, it doesn't make short content occupy the screen. That distinction is what the original code was missing.

Verification

• swift test — added the missing emission test (a declared .medium reaches the node; no detents emits an empty array, which the interpreter reads as full height), 97 total passing
• Emulator, before/after screenshots on the Sheet screen: the full-size sheet went from a ~20% strip to covering the screen, and the medium sheet now rests at half height with the presenting screen visible above it — the two are finally distinguishable.