fix(rpcclient): stop WebDAV mount when the target disconnects

[doronz88]

Problem

The webdav / rpcdav runner blocked on a fresh asyncio.Event() that nothing ever set:

run_in_loop(asyncio.Event().wait())

So when the target went away — device reboot, killed rpcserver, dropped connection — the WebDAV server, the local mount, and the process all stayed up serving errors until the user hit Ctrl-C.

(Mirrors pymobiledevice3's webdav: Close the server when the device disconnects.)

Fix

Wait on the RPC connection instead of a dead event:

• ClientEvent.TERMINATED (already emitted by CoreClient.rpc_call on ConnectionError) unblocks the instant an in-flight WebDAV request fails.
• A periodic liveness probe (getpid, every 5 s) catches a disconnect that happens while the mount is idle and no request is in flight — rpc-project has no background reader, so an idle drop is otherwise invisible.

On disconnect we print Connection to <host> lost; stopping. and run the existing teardown (unmount, stop server, close client). Ctrl-C behaviour is unchanged.

Verification

• New deterministic tests: test_wait_for_disconnect_returns_on_terminated_event (notifier path) and test_wait_for_disconnect_detects_idle_disconnect_via_heartbeat (probe path).
• Full tests/test_webdav.py passes (25 passed) against a live rpcserver_ios; pyright clean.
• Manually verified against the vphone: closing the socket under an idle client is detected within one probe interval (~1 s); a TERMINATED notification returns immediately without waiting for the probe.

https://claude.ai/code/session_01X7BxNXzz1dJmtZngoj5kab