fix(iOS): clean up screen animation mount observers

[OskarEichler]

Problem and fix

ScreenAnimationController registered itself for React Native mount notifications on every transition but never unregistered. Under the New Architecture, RCTSurfacePresenter stores observers in a weak vector whose dead entries are not compacted, so repeated navigation leaves an ever-growing list for every mount notification to scan.

This change:

• registers only when a transition actually needs to wait for a React mount
• makes observer registration idempotent
• unregisters before starting the animation, when the transition ends or is cancelled, and during deallocation
• covers the lifecycle with a regression test that verifies duplicate transitions register once and repeated cleanup removes once

The legacy RCTUIManagerObserverCoordinator path receives the same balanced lifecycle cleanup.

Verification

• New Architecture iOS build-for-testing succeeded
• ScreenAnimationControllerTest: 1/1 passed on an iPhone 16 Pro simulator (iOS 26.5)
• project file validation passed
• clang-format and git diff --check passed

Breaking changes

None. This only balances internal observer registration and removal.