[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

Start by tracing FlightServer cleanup and its allocator lifecycle in the Java FlightRPC implementation. Review the flaky Flight tests described in apache/arrow#38318 and reproduce the cleanup behavior. Done means identifying and fixing the server allocator leak without manually cleaning up child allocators, with the affected tests passing.

Written by the indexing model from the issue text.