chore(types): remove nullability from some children props in unstable/experimental components

[kligarski]

Description

This PR is a follow-up to #3719. We adjust children property to be not nullable for Host components (Tabs, Split and Stack v5) and subviews in Stack v5 implementation.

This change is applied only to unstable/experimental components so it shouldn't be considered a breaking change.

Changes

• wrap ReactNode/ViewProps['children'] in NonNullable for:
  • StackHost
  • TabsHost
  • SplitHost
  • StackHeaderSubview and related places in StackHeaderConfig

Before & after - visual documentation

N/A

Test plan

yarn check-types

Checklist

• For API changes, updated relevant public types.
• Ensured that CI passes

[kligarski]

I decided not to mess with overriding children prop in Fabric specs but it's not a public API so it shouldn't be a problem.

[Copilot]

Pull request overview

This PR tightens TypeScript typings for experimental/unstable Host components by making their children (and related header subview component fields) non-nullable, aligning with the repo’s exactOptionalPropertyTypes work and ensuring these hosts always receive renderable content.

Changes:

• Make children required and NonNullable<...> for TabsHost, StackHost (gamma), and SplitHost (gamma).
• Make Stack v5 Android header subview children required and non-nullable.
• Make Stack v5 Android header config subview Component fields non-nullable.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/components/tabs/host/TabsHost.types.ts	Makes TabsHost children required and non-nullable.
src/components/gamma/stack/host/StackHost.types.ts	Makes StackHost children non-nullable.
src/components/gamma/stack/header/android/StackHeaderSubview.android.types.ts	Makes header subview children required and non-nullable.
src/components/gamma/stack/header/StackHeaderConfig.android.types.ts	Makes toolbar/background subview Component non-nullable.
src/components/gamma/split/SplitHost.types.ts	Makes SplitHost children required and non-nullable.

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