optional signal_shutdown argument to Worker.run

[DawidBudzynsky]

What was changed

Added an optional, keyword-only signal_shutdown argument to Worker.run:

await worker.run(signal_shutdown=[signal.SIGTERM, signal.SIGINT])

When set, the listed OS signals initiate the same graceful shutdown as calling Worker.shutdown(). Handlers are installed for the duration of the run() call and removed on exit (including when run() exits via a fatal worker error).

Implementation notes:

• Uses loop.add_signal_handler where available; the handler sets the existing internal _shutdown_event, i.e. the exact same trigger shutdown() uses.
• Falls back to signal.signal on platforms where add_signal_handler is unsupported (e.g. Windows), scheduling the shutdown via loop.call_soon_threadsafe so a blocked event loop is still woken.
• Previous handlers are restored on exit; restoration tolerates a None previous handler (which signal.signal would otherwise reject).
• Default behavior is unchanged: with no signal_shutdown argument (or an empty sequence), no handlers are installed.

Why?

Workers commonly run in containers/orchestrators (Kubernetes, systemd, etc.) that signal SIGTERM/SIGINT to request shutdown. Previously, translating those signals into a graceful Worker.shutdown() was left entirely to the caller's own boilerplate. This makes the common case a one-line opt-in while keeping the explicit shutdown() path fully working and the default behavior untouched.

Checklist

1. How was this tested:
   Added 7 tests in tests/worker/test_worker.py (all passing locally against a
   dev server):

• default (no arg) behavior is unchanged
• empty sequence is a no-op and leaves existing handlers untouched
• a configured signal triggers graceful shutdown and run() returns cleanly
• any one of multiple registered signals triggers shutdown
• handlers are removed after run() returns
• explicit shutdown() still works when signal_shutdown is registered
• handlers are removed even when run() exits via a fatal worker error

3. Any docs updates needed?
   The new argument is documented in the Worker.run docstring, and a entry was added to CHANGELOG.md. No
   docs.temporal.io changes required.

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