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

Start in Lib/multiprocessing/shared_memory.py at SharedMemory.del and close(), using the Linux reproduction in the issue to observe the exported-buffer failure and leaked file descriptor. Check the linked PRs gh-155007 and gh-155070 before beginning; done means the destructor no longer leaves the descriptor open while preserving the relevant mmap and buffer behavior.