HaEntity describes an entity for device-based discovery, throttles its state (SEN-14) and reports availability (SEN-15); HaSensor is the `sensor` type. HaDevice publishes discovery, availability and states after each connect and after HA's birth message, provides the last will and removes itself from HA. Unit tests with a fake MQTT client. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>