[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

首先跟踪 Java FlightRPC 实现中的 FlightServer 清理过程及其 allocator 生命周期。检查 apache/arrow#38318 中描述的 flaky Flight 测试，并复现清理行为。完成的标准是：在不手动清理子 allocator 的情况下，识别并修复 server allocator 泄漏，并且受影响的测试通过。

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