fix(messaging,ios): register for APNs after Dart Firebase.initializeApp()

[SelaseKay]

Description

firebase_messaging 16.5.0+ skips registerForRemoteNotifications at launch when Firebase is initialized from Dart only (no GoogleService-Info.plist in the iOS app bundle, no native FirebaseApp.configure()).

isAutoInitEnabled is read in didFinishLaunching, before Dart initializeApp(). Without a bundled plist, [FIRMessaging messaging] is nil, the gate reads as NO, and APNs registration never happens. Result: getAPNSToken() stays null, getToken() throws apns-token-not-set, and onTokenRefresh never fires.

This PR re-runs the auto-init check from pluginConstantsForFIRApp: after firebase_core has configured the default app. Launch-time registration is unchanged for apps that still bundle a plist. FirebaseMessagingAutoInitEnabled = NO is still honored (no registration until setAutoInitEnabled(true)).

Related Issues

Fixes #18643

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

• No, this is not a breaking change.

Test plan

• Run an iOS device build of this branch with Dart-only Firebase init (no GoogleService-Info.plist in Copy Bundle Resources).
• getAPNSToken() is non-null and getToken() succeeds without apns-token-not-set.
• Apps that still bundle GoogleService-Info.plist continue to register at launch.
• FirebaseMessagingAutoInitEnabled = NO still skips registration until setAutoInitEnabled(true).

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