[General] Use handler observer API for relation updates

[j-piasecki]

Description

#4179 changed the timing of gesture creation and attachment, after that change configureRelations was being fired before the gesture was created.

This PR updates the relations configuration flow to use the observers API.

Test plan

Example app

[Copilot]

Pull request overview

Fixes a regression introduced by PR #4179: after gesture creation was deferred to a parent effect, configureRelations ran during the child detector's render before the handler existed on the native side, so the first relations call became a no-op. This PR routes relation configuration through the existing observeHandler API so it is deferred until the handler is actually registered, on all three platforms.

Changes:

• Introduces useGestureRelationsUpdater (a useMemo + useEffect) and switches NativeDetector, VirtualDetector, and InterceptingGestureDetector to use it instead of calling configureRelations during render.
• Refactors configureRelations in detectors/utils.ts to return a Map<handlerTag, GestureRelations> rather than pushing directly to the native side, so the hook controls when relations are applied.
• Updates the native configureRelations entry points (Android RNGestureHandlerModule, iOS RNGestureHandlerManager, web RNGestureHandlerModule.web.ts) to subscribe via observeHandler / observeHandlerWithTag:, cancelling the observation after the first delivery.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/react-native-gesture-handler/src/v3/detectors/useGestureRelationsUpdater.ts	New hook that memoizes the relations map and applies it in an effect.
packages/react-native-gesture-handler/src/v3/detectors/NativeDetector.tsx	Replaces inline configureRelations call with the new hook.
packages/react-native-gesture-handler/src/v3/detectors/VirtualDetector/VirtualDetector.tsx	Same hook replacement for virtual detector.
packages/react-native-gesture-handler/src/v3/detectors/VirtualDetector/InterceptingGestureDetector.tsx	Replaces conditional inline call with the hook (now safe with undefined gesture).
packages/react-native-gesture-handler/src/v3/detectors/utils.ts	configureRelations no longer dispatches to native; returns a tag→relations map and threads it through traversal.
packages/react-native-gesture-handler/src/RNGestureHandlerModule.web.ts	Replaces synchronous lookup + early return with NodeManager.observeHandler flow.
packages/react-native-gesture-handler/apple/RNGestureHandlerManager.mm	Uses registry observeHandlerWithTag: instead of direct handlerWithTag:.
packages/react-native-gesture-handler/android/src/main/java/com/swmansion/gesturehandler/react/RNGestureHandlerModule.kt	Uses registry observeHandler instead of getHandler + early return.

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