gh-146313: Fix multiprocessing ResourceTracker deadlock after os.fork() (GH-146316)

`ResourceTracker.__del__` (added in gh-88887 circa Python 3.12) calls
os.waitpid(pid, 0) which blocks indefinitely if a process created via os.fork()
still holds the tracker pipe's write end. The tracker never sees EOF, never
exits, and the parent hangs at interpreter shutdown.

Fix with two layers:

- **At-fork handler.** An os.register_at_fork(after_in_child=...)
  handler closes the inherited pipe fd in the child unless a preserve
  flag is set. popen_fork.Popen._launch() sets the flag before its
  fork so mp.Process(fork) children keep the fd and reuse the parent's
  tracker (preserving gh-80849). Raw os.fork() children close the fd,
  letting the parent reap promptly.

- **Timeout safety-net.** _stop_locked() gains a wait_timeout
  parameter. When called from `__del__`, it polls with WNOHANG using
  exponential backoff for up to 1 second instead of blocking
  indefinitely. The at-fork handler makes this unreachable in
  well-behaved paths; it remains for abnormal shutdowns.

Co-authored-by: Itamar Oren <itamarost@gmail.com>