fix(iOS, Stack v5): Refactor header coordination to hide header when no headerConfig is provided

[kmichalikk]

Closes https://github.com/software-mansion/react-native-screens-labs/issues/1699

Description

Currently on v5 when no headerConfig is provided native header is rendered empty, with back button present. Additionally, the header is set to large variant by default, and that looks bad. This PR hides the header entirely when no config is provided, matching the behavior in v4.

Changes

• moved the header coordinator wiring header config earlier so we can catch the fact when screen viewWillAppear fires
• added method for setting header visibility
• added screen.viewWillAppear that hides the header if no config provider is present

Before & after - visual documentation

n/a

Test plan

Use simple navigation SFT, hide the header on one screen and navigate; use test-stack-header-subviews-ios, hide / show the header via prop updates. Verify that it hides.

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 Stack v5 header coordination so screens without a header configuration hide the native navigation bar, matching v4 behavior.

Changes:

• Wires header coordinators during child mounting.
• Updates navigation-bar visibility during screen appearance and configuration changes.
• Clears header coordinator resources when configurations unmount.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
ios/stack/screen/RNSStackScreenHeaderCoordinator.mm	Centralizes navigation-bar visibility handling.
ios/stack/screen/RNSStackScreenHeaderCoordinator.h	Exposes the visibility update method.
ios/stack/screen/RNSStackScreenController.mm	Updates visibility before screen appearance.
ios/stack/screen/RNSStackScreenComponentView.mm	Wires and clears header coordination during mounting.
ios/stack/header/RNSStackHeaderConfigComponentView.mm	Uses the coordinator assigned by the parent screen.
ios/stack/header/RNSStackHeaderConfigComponentView.h	Adds the weak coordinator property.

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