NameError: Value not defined in two threads-only pool tests when ctypes is unavailable

In Lib/test/_test_multiprocessing.py, both
test_imap_and_imap_unordered_when_buffer_is_full and
test_imap_and_imap_unordered_with_buffersize_when_buffer_is_full create a
shared value with a module-level Value:

last_produced_task_arg = Value("i")

But Value is only conditionally imported at the top of the module:

try:
    from multiprocessing.sharedctypes import Value, copy
    HAS_SHAREDCTYPES = True
except ImportError:
    HAS_SHAREDCTYPES = False

So when ctypes/sharedctypes are unavailable (HAS_SHAREDCTYPES == False), the
name Value is undefined. Unlike the other sharedctypes-based tests in this
file (which use @unittest.skipUnless(HAS_SHAREDCTYPES, ...)), these two tests
have no such guard, so they run and raise:

NameError: name 'Value' is not defined. Did you mean: 'self.Value'?

Reproduced in verbose mode as 4 errors (2 tests x {imap, imap_unordered})
when running test_multiprocessing_forkserver.test_threads on a build without
ctypes.

These tests are threads-only (they call skipTest() for other backends), so
self.Value resolves to multiprocessing.dummy.Value and does not depend on
ctypes. Replacing Value("i") with self.Value("i", 0):

• fixes the NameError on builds without ctypes, and
• matches how the rest of the suite creates shared values. Since both
  forms initialize the value to zero, there is no behavior change when
  ctypes is available.

Proposed fix:

- last_produced_task_arg = Value("i")
+ last_produced_task_arg = self.Value("i", 0)

This is my first contribution to CPython. I used Claude and opencode to help
me understand the codebase, and vim to edit the file. I reviewed and tested
the change myself; please let me know if I missed any process step

Linked PRs

• gh-155333

从 Lib/test/_test_multiprocessing.py 开始，检查 test_imap_and_imap_unordered_when_buffer_is_full 和 test_imap_and_imap_unordered_with_buffersize_when_buffer_is_full。在不包含 ctypes 的构建上运行 test_multiprocessing_forkserver.test_threads；当两个测试都避免出现 NameError 且测试运行通过时，即表示完成。