The energy-cost device has only sensors (no online/offline heartbeat), but the
discovery config still declared an availability topic that nothing ever publishes
'online' to — so Home Assistant marked every energy-cost entity 'unavailable'
even though the state topic was being published (visible in MQTT Explorer).
- DeviceInfo gains provides_availability (default True, keeps Modbus behavior).
- _energy_cost_provider sets provides_availability=False.
- build_discovery_payload only emits availability/availability_mode for devices
that publish a heartbeat; HA treats the rest as always-available.
- Test asserts energy-cost configs omit availability.
tliu93