Issue #3538825: ApiExceptionSubscriber prevents ExceptionLoggingSubscriber from running

ApiExceptionSubscriber and core ExceptionLoggingSubscriber both subscribed to KernelEvents::EXCEPTION at priority 50. ApiExceptionSubscriber sets a response, which stops event propagation, so the logging subscriber never ran and exceptions on Canvas API routes were never logged.

Lowers ApiExceptionSubscriber to priority -50 so it runs after the logging subscriber (50) but still before ExceptionJsonSubscriber (-75). Adds a kernel test asserting ApiExceptionSubscriber has a strictly lower KernelEvents::EXCEPTION priority than ExceptionLoggingSubscriber.