debugAppCheckActivatorOverride top-level property
Test seam: when non-null, appInitAppCheck uses this in place of the real
FirebaseAppCheck.instance activation (_activateFirebaseAppCheck), so the
bounded/non-critical wrapper can be unit-tested without the platform
singleton (unavailable in the test VM). Null in production; reset to null in
test tearDown.
Implementation
@visibleForTesting
Future<void> Function(AppCheckConfig config)? debugAppCheckActivatorOverride;