gh-156333: rebuild the proactor self-pipe on EOF instead of busy-looping

[aidaodedjl]

Summary

On Windows, BaseProactorEventLoop wakes itself through a self-pipe — a loopback TCP socketpair created by socket.socketpair(). If that connection reaches a clean EOF while the loop is running — for example the OS tears the idle loopback connection down across a power/session state change — _loop_self_reading re-armed recv() on the dead socket, which completed immediately and rescheduled the callback forever: one core pinned at 100% CPU, no exception raised, nothing logged, the process never recovers. Reproduced deterministically on main (3.16.0a0, self-built): 346,702 re-arms of _loop_self_reading during a 3-second idle sleep (a graceful loop._csock.shutdown(socket.SHUT_WR) models the OS teardown).

At EOF f.result() returns b'', which is not an exception, so control fell through to the else branch and armed a read that could never block again.

Fix: when the recv result is empty, rebuild the socketpair instead of re-arming on the dead one:

• allocate the replacement pair first, so a failure inside the rebuild leaves the previous state untouched and the error propagates instead of being swallowed by the catch-all handler;
• re-register signal.set_wakeup_fd() on the new socket before closing the old sockets, mirroring the ordering used by close();
• arm the next read on the new socket, so cross-thread wakeups (call_soon_threadsafe) keep working after the rebuild.

Verification (real Windows machine, self-built 3.16.0a0 from the commit this PR is based on)

• The issue's reproducer: 346,702 re-arms / 1.42s CPU during a 3s sleep before → 0 re-arms / 0.00s CPU after; the loop still completes cross-thread wakeups after the rebuild.
• Full test_asyncio suite: 35/35 files, 2,625 tests, 0 failures (both new tests included).
• Both new regression tests were verified to fail on unpatched main (git checkout of the pristine proactor_events.py, tests re-run: mock test FAIL, functional test FAIL), then pass with the patch.

Out of scope / follow-ups

• The same OS teardown can also surface as ConnectionResetError from the pending recv instead of a clean EOF; that pre-existing path is not handled here.
• BaseSelectorEventLoop._read_from_self (used by WindowsSelectorEventLoop) has the same EOF busy-loop shape: measured 582,692 _read_from_self calls during a 3s idle sleep on the same machine/reproducer shape. Will be reported separately.

• Issue: asyncio: ProactorEventLoop busy-loops at 100% CPU forever when the self-pipe socketpair reaches EOF #156333