fix(push-notifications): resolve unregister() only after FCM token deletion completes on Android

[ciccilleju]

On Android, unregister() calls FirebaseMessaging.deleteToken() and resolves the JavaScript call immediately, before token deletion finishes. Even when the application awaits unregister() before calling register(), getToken() can still return the cached token. FCM then invalidates that token, leaving the backend with a token that fails with UNREGISTERED and preventing subsequent push delivery.

This change resolves unregister() from deleteToken()'s completion listener and rejects the call if deletion fails. The change is limited to Android; the iOS implementation is unchanged.

Fixes #2593.

Reproduction

1. Register for push notifications and capture the token from the registration event.
2. Run the following sequence without adding a delay:

await PushNotifications.unregister();
await PushNotifications.register();

3. Capture the next token from the registration event. With the current implementation, the previous token can be returned while its deletion is still pending; sending to that token subsequently fails with UNREGISTERED.
4. With this change, the unregister() promise resolves only after deletion succeeds, so the awaited register() starts after that deletion has completed. A deletion failure rejects unregister() instead of reporting success.

Validation

• An equivalent fix is applied locally with patch-package to @capacitor/push-notifications 8.1.3, in an app using Capacitor 8.5.2.
• Android plugin Java compilation passed with Java 21:

./gradlew :capacitor-push-notifications:compileDebugJavaWithJavac

• This verifies compilation of the local equivalent fix. End-to-end FCM delivery and the deletion-failure path were not tested in this check.
• Repository CI has not run yet: the pull request workflows currently show action_required.

[ciccilleju]

This also addresses #2593. The issue occurs even when awaiting unregister() before calling register(), because the promise currently resolves before FCM token deletion completes. I have updated the description with the awaited reproduction sequence and validation details.

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