[Java][FlightRPC] FlightServer can leak during cleanup

### Describe the bug, including details regarding any error messages, version, and platform.

In apache/arrow#38318 , a few Flight tests were flakey due to occasionally leaking during cleanup. The leaks were coming from the server's allocator specifically. A workaround is to cleanup child allocators, however it would be best to determine the root cause of the leak and not have to manually clean-up child allocators.

### Component(s)

FlightRPC, Java

Comienza rastreando la limpieza de FlightServer y el ciclo de vida de su allocator en la implementación de Java FlightRPC. Revisa las pruebas flaky de Flight descritas en apache/arrow#38318 y reproduce el comportamiento de limpieza. El trabajo estará terminado cuando se haya identificado y corregido la fuga del allocator del servidor sin limpiar manualmente los allocators secundarios, y las pruebas afectadas pasen.

Escrito por el modelo de indexación a partir del texto del issue.