Pre-existing pytest teardown flakiness (paho-mqtt loop_stop)

FIX 2026-09-06 (code landed, unverified — no brain venv here to run pytest): zigbee_mqtt.ZigbeeIngest.stop() now (1) calls disconnect() before loop_stop() so paho winds its loop down cleanly and stops reconnecting, (2) wraps both in try/except so teardown never raises, (3) joins the ingest _run thread (timeout=2s, skips if current thread) so stop() is synchronous — a lingering daemon thread processing one last message was the flake source, (4) nulls _thread/_client. — ORIGINAL: pytest teardown flakiness from paho-mqtt loop_stop.