Windows: watchdog self-kill must os._exit, not no-op sys.exit from a bg thread

[danielhertz1999-bit]

Problem

The liveness watchdog can detect a wedged daemon but fails to actually kill it on Windows, so the daemon stays down until a manual taskkill.

The watchdog runs in a background daemon thread (name="iai-liveness-watchdog", daemon/__init__.py). When it decides to kill, _self_kill() in daemon/_watchdog.py branches on hasattr(signal, "SIGKILL"):

if hasattr(signal, "SIGKILL"):
    os.kill(os.getpid(), signal.SIGKILL)   # POSIX: signals the whole process
else:
    sys.exit(1)                            # Windows: NO-OP from a bg thread

On Windows there is no SIGKILL, so it falls to sys.exit(1) — which only raises SystemExit in the calling thread. From the background watchdog thread that unwinds the thread, not the process, so the wedged daemon keeps running. Task Scheduler's <MultipleInstancesPolicy>IgnoreNew</MultipleInstancesPolicy> then treats the task as still "Running" and silently refuses to start a replacement, so the daemon stays down until someone kills the zombie by hand.

Observed in the wild: watchdog logs daemon_wedge_kill pid=<pid> (intent recorded) but the process with that pid is still alive minutes later.

Fix

Use os._exit(1) on the no-SIGKILL path. It terminates the whole process immediately from any thread and bypasses interpreter shutdown — which is what you want here, since normal cleanup could itself hang on the same wedge the watchdog is escaping. POSIX behavior (SIGKILL) is unchanged.

Test

The three existing _self_kill tests are all gated on @_REQUIRES_SIGKILL, so they skip on Windows — which is precisely why the no-op path shipped uncaught. The new test simulates the no-SIGKILL branch (via monkeypatch.delattr(signal, "SIGKILL")) so it runs on every platform, and asserts _self_kill calls os._exit and never sys.exit/os.kill. Verified both ways: passes on the fix, fails against the pre-fix source with Failed: sys.exit is a no-op from the watchdog thread.

Windows-only behavior change; POSIX paths are unaffected.