feat(ios): migrate iOS to UIScene lifecycle

[Lyokone]

Description

Migrate all iOS example apps to UIScene lifecycle and add UIScene support to firebase_auth.

Example apps (17 apps + tests app):

• Added UIApplicationSceneManifest to all Info.plist files with FlutterSceneDelegate
• ObjC apps: updated AppDelegate to use FlutterImplicitEngineDelegate pattern (plugin registration via didInitializeImplicitFlutterEngine:)
• Swift apps: no AppDelegate changes needed (Flutter handles migration)

firebase_auth plugin:

• Added FlutterSceneLifeCycleDelegate conformance (conditional, backwards-compatible)
• Added addSceneDelegate: registration in registerWithRegistrar:
• Added scene:openURLContexts: for OAuth/custom-scheme redirects under UIScene
• Fixed presentationAnchorForAuthorizationController: to use scene-aware window lookup instead of deprecated UIApplication.keyWindow

Related Issues

Fixes #18026

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

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

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[araful39]

Xcode build done. 11.4s
Failed to build iOS app
Swift Compiler Error (Xcode): Compiling for iOS 13.0, but module 'FirebaseCrashlytics' has a minimum deployment target of iOS 15.0: /Users/softvence/Araful/felicerosee_new_app/build/ios/Debug-iphonesimulator/FirebaseCrashlytics/FirebaseCrashlytics.framework/Modules/FirebaseCrashlytics.swiftmodule/arm64-apple-ios-simulator.swiftmodule

Could not build the application for the simulator.
Error launching application on iPhone 17 Pro.

[araful39]

error code and solve code

[MirceaX2Mobile]

@Lyokone @SelaseKay I can't find this header in the latest stable flutter engine. And it always shows "Plugin FLTFirebaseAuthPlugin uses deprecated application lifecycle events." Are we sure about this?

Shouldn't be #if __has_include(<Flutter/FlutterSceneLifeCycle.h>)?
https://api.flutter.dev/ios-embedder/protocol_flutter_scene_life_cycle_delegate-p.html