fix(messaging,ios): do not suppress forwarded local notifications

[SelaseKay]

Description

On iOS, willPresentNotification called the shared completion handler with UNNotificationPresentationOptionNone whenever this plugin had not replaced another delegate. UIScene apps that assign UNUserNotificationCenter.delegate to AppDelegate forward that callback to every plugin. Messaging answering None hid foreground banners posted by other plugins, including flutter_local_notifications, after 16.7.0.

The handler is now called only when:

• another delegate was replaced and must receive the callback, or
• this plugin is the notification center delegate, or
• Flutter forwarded a remote FCM notification (aps and gcm.message_id) and the app opted into foreground presentation via setForegroundNotificationPresentationOptions.

A forwarded local notification is left for the plugin that posted it. The same split is applied to didReceiveNotificationResponse so a tap on that local notification is not consumed. APNs registration from #18620 is unchanged.

Related Issues

Fixes #18699

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

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.

Test plan

• UIScene app, AppDelegate owns UNUserNotificationCenter.delegate, no setForegroundNotificationPresentationOptions: a data-only FCM message in the foreground still reaches onMessage, and a local notification posted from it is displayed.
• Same setup with FLTFirebaseMessagingPlugin.configureNotificationCenterDelegate() called before the host sets the delegate.
• Fresh install (app deleted): getToken() resolves and an APNs token is received ([firebase_messaging]: 16.5.0 never receives APNs token under UIScene (apns-token-not-set) — regression from 16.4.3 #18555 / fix(messaging,ios): register for APNs when UIScene plugins miss launch callbacks #18620).
• setForegroundNotificationPresentationOptions(alert: true) still shows an FCM notification message in the foreground.
• An app that never sets presentation options still does not show an FCM notification banner.
• Tapping a local notification still reaches the other plugin. Tapping a remote FCM notification still fires onMessageOpenedApp.

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