[Python][FlightRPC] FlightServerBase is never freed, leaking the server and everything it references #50684

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

A FlightServerBase, even after shutdown(), wait() and dropping the last user reference, doesn't seem to be completely freed. It stays alive and leaks.

Applications that create a Flight server per unit of work (one per build, per request batch, per test) accumulate one zombie server per cycle.

$ uv run python
Python 3.14.2 (main, Jan 14 2026, 19:38:07) [Clang 21.1.4 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import gc
... import pyarrow.flight as fl
... 
... class Server(fl.FlightServerBase):
...     pass
... 
... for _ in range(100):
...     server = Server("grpc://127.0.0.1:0")
...     server.payload = bytearray(8 * 1024 * 1024)
...     server.shutdown()
...     server.wait()
...     del server
...     gc.collect()
... 
... print(sum(1 for o in gc.get_objects() if isinstance(o, Server)))
... 
100
>>> import pyarrow
>>> pyarrow.__version__
'25.0.0'

RSS grows ~8 MB per iteration, tracking the payload one for one. The same loop with a plain Python object instead of a Flight server keeps RSS flat and prints 0.

Without a payload the loop still leaks about 66 kB RSS and 12 to 16 MB of virtual address space per iteration. An equivalent C++ loop creating and shutting down a Flight server shows no growth, so this points at an issue in the Python bindings.

Component(s)

FlightRPC, Python