fix(iOS): remove FormSheet scroll view KVO observer synchronously on invalidate

[phewphewb]

Description

Fixes #4090

After dismissing FormSheet, pushed screen ScrollView is clipped to sheet height.
FormSheet KVO observer not removed before scroll view is reused by the next screen.

Changes

• Remove the FormSheet scroll view KVO observer synchronously at the start of -invalidateImpl, instead of inside the dispatch_async block introduced in feat(iOS, Stack v4): Use new invalidation callback on Fabric #3728
• Keep deferring _controller - nil to the async block (unchanged)
• Add Test4090 as a minimal repro for dismissing a form sheet and immediately pushing a screen with a full-height ScrollView

Before & after - visual documentation

Before

Simulator.Screen.Recording.-.iPhone.17.Pro.-.2026-05-22.at.22.59.32.2.mov

After

Simulator.Screen.Recording.-.iPhone.17.Pro.-.2026-05-24.at.18.38.05.mov

Test plan

Checklist

• Included code example that can be used to test this change.
• For visual changes, included screenshots / GIFs / recordings documenting the change.
• For API changes, updated relevant public types.
• Ensured that CI passes

[phewphewb]

@kkafar @t0maboro @kligarski