Fix ASGI SIGTERM handler to chain to host server and skip lifespan scope - #240

Fixes #236.

Changes

• Reject lifespan and non-http scopes before installing the SIGTERM handler so lifespan probes do not displace the host server's signal handler.
• Capture the existing SIGTERM handler from both getattr(loop, '_signal_handlers', {}).get(signal.SIGTERM) (used by loop.add_signal_handler hosts such as Hypercorn) and signal.getsignal(signal.SIGTERM) (used by signal.signal hosts such as Uvicorn) before registering _on_sigterm.
• Chain to the captured handler from _on_sigterm after notifying active ReceiveChannel instances.
• Add unit tests in tests/server.py covering loop.add_signal_handler chaining (including callback arguments), signal.signal chaining, and lifespan / non-http scopes.

Related: #226.