[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

首先追蹤 Java FlightRPC 實作中的 FlightServer 清理流程及其 allocator 生命週期。檢視 apache/arrow#38318 中描述的 flaky Flight 測試，並重現清理行為。完成的標準是：在不手動清理子 allocator 的情況下，識別並修正 server allocator 洩漏，且受影響的測試通過。

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