refactor(iOS, FormSheet v5): Introduce AppearanceApplicator

[t0maboro]

Description

Introducing the AppearanceApplicator class, which is responsible for the configuration of UISheetPresentationController. Moving already implemented configuration from the HostView's scope.
Note: This might not be the final shape, because the Host should only receive props and notify the controller, but it seems to be independent of introducing the AppearanceApplicator concept, and it can be implemented in the stacked PR.

Note

I know that AppearanceApplicator should be stateless, but in this PR, I just want to introduce the concept and move the implementation to a dedicated class, preserving all functionalities. I want to move both AppearanceApplicator & AppearanceCoordinator to the Controller level in a follow-up PR.

Changes

• Introduced AppearanceApplicator following the SplitView's implementation.
• Moved the logic related to sheet configuration from Host to AppearanceApplicator

Before & after - visual documentation

N/A - refactor

Test plan

Go through available SFTs for sheets.

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

[Copilot]

Pull request overview

Refactors iOS FormSheet (gamma) configuration by extracting UISheetPresentationController setup logic out of RNSFormSheetHostComponentView into a new dedicated RNSFormSheetAppearanceApplicator class, mirroring the SplitView pattern.

Changes:

• Introduced new RNSFormSheetAppearanceApplicator class encapsulating sheet configuration and the _initialDetentApplied state.
• Removed inline updateConfiguration and related ivars from the host view; promoted several ivars to readonly properties (with a detents accessor) so the applicator can read host state.
• Wired the applicator into finalizeUpdates and reset of initial-detent flag on reopen.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
ios/gamma/modals/form-sheet/RNSFormSheetAppearanceApplicator.h	New header declaring the applicator interface.
ios/gamma/modals/form-sheet/RNSFormSheetAppearanceApplicator.mm	New implementation containing the moved configuration logic and initial-detent tracking.
ios/gamma/modals/form-sheet/RNSFormSheetHostComponentView.h	Exposes host props as readonly properties and a detents accessor via a class extension.
ios/gamma/modals/form-sheet/RNSFormSheetHostComponentView.mm	Removes inline configuration, instantiates the applicator, and delegates updates to it.

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