Fix the issues found in the review of 1.0.3

[DAB-LABS]

I had 1.0.3 reviewed again, this time by someone who drove the real network path our tests fake. They found two things.

First, if a device's network socket went bad (the interface bounced, the host got a new address), the library kept using the dead socket and every request timed out until the device was reloaded. The original opened a fresh socket per call, so it healed on its own. Second, a hostname that did not resolve made the library wait out the full timeout and, for pings, silently send nothing, where the original failed at once with the error Home Assistant maps to "invalid host".

Both are fixed, and we carried a fix from an open upstream pull request for the A2 air quality sensor, which its author tested on a real unit, but we have no way to confirm. Tested on Python 3.13 and 3.14, 269 tests, plus a live run against an RM4 Pro that included a forced socket failure and a bad hostname.

Coding assistant's notes on this change:

The datagram protocol now puts socket errors on the receive queue, so a waiting request fails at once with that OSError for send failures (no route, address gone), and any request that fails for a network reason drops the endpoint so the next call opens a fresh socket. ICMP port unreachable is treated as silence, since the original's unconnected socket never saw it, so a host that is up but is not a Broadlink device still times out as before rather than surfacing a new error type. scan(), ping() and send_setup_packet() resolve their destination once with loop.getaddrinfo and let socket.gaierror propagate; nothing resolves on the event loop any more.

The A2 request frame now follows the SP4/LB1 layout (length 12, four-byte data length), which is byte for byte the packet upstream mjg59#826 tested; it is the one oracle case re-recorded on purpose, noted in the harness, with Co-authored-by credit. Also in this change: xdiscover() closes the scan() generator it wraps; captured_at no longer takes part in CapturedSignal equality; async generator functions are annotated AsyncGenerator, which has the aclose() we call; the TICK docstring matches the README; a new loopback test module drives the real endpoint including 20 concurrent requests and the ICMP path; the README says which errors discover() and hello() raise. Still deferred to 1.1: py.typed and mypy in CI, and moving the CLIs into the package so they are linted.