docs(app_check): document native configure() workaround on Apple

[SelaseKay]

Description

Documents a known limitation of firebase_app_check on Apple platforms: if an app calls FirebaseApp.configure() in AppDelegate (before plugin registration), Dart activate() providerApple is ignored on physical devices and the SDK attests with DeviceCheck. This is because the Apple SDK requires AppCheck.setAppCheckProviderFactory to be called before configure().

This PR does not change plugin behavior. It documents the supported FlutterFire path (Firebase.initializeApp() only) and the native workaround for apps that must configure Firebase in AppDelegate.

Related Issues

• Fixes [firebase_app_check] iOS: activate() providers are silently ignored when FirebaseApp.configure() runs in AppDelegate (firebase-ios-sdk >= 12.15.0 default provider factory) #18613

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• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

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• Yes, this is a breaking change.
• No, this is not a breaking change.

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