unittest mocks don't get applied to children with forkserver multiprocessing start method

Bug report

Bug description:

import multiprocessing
import unittest
from unittest import mock


class Hello(unittest.TestCase):
    def _test_world(x):
        print(x, World().test())

    def test_world(self):
        # OK: multiprocessing.set_start_method("fork")
        # NOK: multiprocessing.set_start_method("forkserver")

        with mock.patch.object(World, "test", return_value=42):
            with multiprocessing.Pool(5) as pool:
                pool.map(Hello._test_world, range(5))


class World:
    def test(self):
        return 1

With "fork":

test.py 0 42
1 42
2 42
3 42
4 42

With "forkserver":

test.py 0 1
1 1
2 1
4 1
3 1

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-152881

從 issue 中的 reproducer 開始，著重於 unittest.mock.patch.object、multiprocessing.Pool 和 forkserver 啟動方法。比較其在 fork 和 forkserver 下的行為，然後檢視連結的 PR gh-152881；完成的標準是，已解決所回報的 mock 行為在 forkserver 下的問題，同時不會導致 fork 回歸。