`test_multiprocessing_spawn.test_processes` flaky tests on Windows

Bug report

Seen on Windows free threading build, but not free threading specific:

https://github.com/python/cpython/actions/runs/13606907022/job/38039459160?pr=130732

This is in Windows specific code. Not yet sure if this is free threading specific.

ERROR: test_async_error_callback (test.test_multiprocessing_spawn.test_processes.WithProcessesTestPoolWorkerErrors.test_async_error_callback)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\a\cpython\cpython\Lib\test\_test_multiprocessing.py", line 3101, in test_async_error_callback
    p.close()
    ~~~~~~~^^
  File "D:\a\cpython\cpython\Lib\multiprocessing\pool.py", line 652, in close
    self._change_notifier.put(None)
    ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "D:\a\cpython\cpython\Lib\multiprocessing\queues.py", line 394, in put
    self._writer.send_bytes(obj)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^
  File "D:\a\cpython\cpython\Lib\multiprocessing\connection.py", line 206, in send_bytes
    self._send_bytes(m[offset:offset + size])
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\a\cpython\cpython\Lib\multiprocessing\connection.py", line 293, in _send_bytes
    raise ValueError("concurrent send_bytes() calls "
                     "are not supported")
ValueError: concurrent send_bytes() calls are not supported

ERROR: test_traceback (test.test_multiprocessing_spawn.test_processes.WithProcessesTestPool.test_traceback)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\a\cpython\cpython\Lib\test\_test_multiprocessing.py", line 2974, in test_traceback
    with self.Pool(1) as p:
         ~~~~~~~~~^^^
  File "D:\a\cpython\cpython\Lib\multiprocessing\pool.py", line 739, in __exit__
    self.terminate()
    ~~~~~~~~~~~~~~^^
  File "D:\a\cpython\cpython\Lib\multiprocessing\pool.py", line 657, in terminate
    self._terminate()
    ~~~~~~~~~~~~~~~^^
  File "D:\a\cpython\cpython\Lib\multiprocessing\util.py", line 216, in __call__
    res = self._callback(*self._args, **self._kwargs)
  File "D:\a\cpython\cpython\Lib\multiprocessing\pool.py", line 690, in _terminate_pool
    change_notifier.put(None)
    ~~~~~~~~~~~~~~~~~~~^^^^^^
  File "D:\a\cpython\cpython\Lib\multiprocessing\queues.py", line 394, in put
    self._writer.send_bytes(obj)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^
  File "D:\a\cpython\cpython\Lib\multiprocessing\connection.py", line 206, in send_bytes
    self._send_bytes(m[offset:offset + size])
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\a\cpython\cpython\Lib\multiprocessing\connection.py", line 293, in _send_bytes
    raise ValueError("concurrent send_bytes() calls "
                     "are not supported")
ValueError: concurrent send_bytes() calls are not supported

test.test_multiprocessing_spawn.test_processes と Lib/test/_test_multiprocessing.py の失敗するケースから始め、次に Lib/multiprocessing/pool.py、queues.py、connection.py をたどって Windows での失敗を追跡します。Windows で失敗を再現し、なぜ concurrent な send_bytes 呼び出しが発生するのかを特定します。対象のテストが断続的に失敗しなくなり、その動作が適切なテストでカバーされれば完了です。