* [Android] Bug fixes:
  - Fixes group notification sticking in running state when cancelling tasks from holding queue
  - Fixes `ConcurrentModificationException` in `canSendCancellation` when processing concurrent status updates
* [iOS] Bug fixes:
  - Fixes group notification sticking in running state when cancelling tasks from holding queue
  - Serializes access to group notification registry to prevent `EXC_BAD_ACCESS` race condition under concurrent status updates
* [General] Defer `JsonProcessor` idle shutdown while requests are pending to prevent unexpected background isolate shutdown errors