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>
