feat(messaging,ios): migrate example iOS runner from ObjC to Swift

[SelaseKay]

Description

Migrates the firebase_messaging example iOS runner from the old Objective-C template to Swift, matching what flutter create generates and the UIScene migration guide.

The example already used FlutterSceneDelegate in Info.plist. This change replaces AppDelegate.h / AppDelegate.m / main.m with AppDelegate.swift that:

• conforms to FlutterImplicitEngineDelegate and registers plugins in didInitializeImplicitFlutterEngine
• still calls FLTFirebaseMessagingPlugin.configureNotificationCenterDelegate() in didFinishLaunchingWithOptions (Apple requires the notification-center delegate before launch returns)
• does not add a custom SceneDelegate.swift — Info.plist continues to use FlutterSceneDelegate directly

Also adds a Swift snippet next to the existing Objective-C one in the plugin README.

This supersedes #18317 by @champ96k, which started this migration. That PR had unresolved review feedback (UIScene AppDelegate adoption, no custom SceneDelegate, keep FlutterSceneDelegate in Info.plist) and had drifted from main (including the later configureNotificationCenterDelegate requirement). This PR applies those review notes on current main.

Related Issues

• Fixes 🐛 [firebase_massaging] Example flutter ios build is too old #13059
• Supersedes fix(firebase_messaging): migrate example iOS project from ObjC to Swift #18317

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• 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

Does your PR require plugin users to manually update their apps to accommodate your change?

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

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