fix(test): Use IMPORTANCE_HIGH for test notifications

IMPORTANCE_DEFAULT notifications may be silently delivered on some
emulators, causing the notification to not appear in the shade
when Maestro swipes down to find it.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: romtsn

sentry-android-integration-tests/sentry-uitest-android-critical/src/main/java/io/sentry/uitest/android/critical/NotificationHelper.kt

@@ -21,7 +21,7 @@ object NotificationHelper {
     // Create notification channel for Android 8.0+ (API 26+)
     if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
       val channel =
-        NotificationChannel(CHANNEL_ID, "Notifications", NotificationManager.IMPORTANCE_DEFAULT)
+        NotificationChannel(CHANNEL_ID, "Notifications", NotificationManager.IMPORTANCE_HIGH)
       channel.description = "description"
       notificationManager.createNotificationChannel(channel)
     }