multiprocessing SharedMemory.__del__ can fail, leak file descriptor

Bug report

Bug description:

When a SharedMemory object goes out of scope, its __del__ method calls self.close() to clean it up. This can break if there is still a buffer exported from its mmap instance. It tries to close the mmap before the fd, so in this case it will leave the file descriptor open:

>>> from multiprocessing.shared_memory import SharedMemory
>>> import os
>>> def shview(size=4096):
...     shmem = SharedMemory(create=True, size=size)
...     print("fd is:", shmem._fd)
...     return shmem.buf.cast('i')
...     
>>> mv = shview()
fd is: 5
Exception ignored while calling deallocator <function SharedMemory.__del__ at 0x7f686264d640>:
Traceback (most recent call last):
  File "/usr/lib64/python3.14/multiprocessing/shared_memory.py", line 189, in __del__
    self.close()
  File "/usr/lib64/python3.14/multiprocessing/shared_memory.py", line 232, in close
    self._mmap.close()
BufferError: cannot close exported pointers exist
>>> os.fstat(5).st_size  # fd still open
4096

I think a better option for __del__ would be to close the fd and leave the mmap & buf objects alone, to be cleaned up as normal by reference counting or GC:

>>> class SharedMemoryFix(SharedMemory):
...     def __del__(self):
...         if self._fd >= 0:
...             os.close(self._fd)
...             self._fd = -1
...             
>>> def shview(size=4096):
...     shmem = SharedMemoryFix(create=True, size=size)
...     print("fd is:", shmem._fd)
...     return shmem.buf.cast('i')
...     
>>> mv = shview()
fd is: 7
>>> os.fstat(7)
Traceback (most recent call last):
  File "<python-input-8>", line 1, in <module>
    os.fstat(7)
    ~~~~~~~~^^^
OSError: [Errno 9] Bad file descriptor

(I'm thinking about the POSIX side here. IDK if the behaviour on Windows needs to be different, though I guess it can rely on the mmap's dealloc too)

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-155007
• gh-155070

Bắt đầu trong Lib/multiprocessing/shared_memory.py tại SharedMemory.del và close(), sử dụng bản tái hiện trên Linux trong issue để quan sát lỗi khi xuất buffer và file descriptor bị rò rỉ. Kiểm tra các PR được liên kết gh-155007 và gh-155070 trước khi bắt đầu; hoàn thành có nghĩa là destructor không còn để descriptor mở, đồng thời vẫn giữ nguyên hành vi liên quan của mmap và buffer.