HA Discovery fix: energy-cost sensors are always-available

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.

app/services/ha_discovery.py

@@ -133,14 +133,21 @@ def build_discovery_payload(
         "unique_id": _unique_id(entity),
         "name": entity.name,
         "state_topic": state_t,
-        "availability": [{"topic": avail_topic}],
-        "availability_mode": "all",
         "device": {
             "identifiers": list(entity.device.identifiers),
             "name": entity.device.name,
         },
     }
 
+    # Only declare an availability topic for devices that actually publish an
+    # online/offline heartbeat (e.g. Modbus, via its "online" binary_sensor).
+    # Devices without a heartbeat (e.g. energy-cost) omit availability so HA
+    # treats their entities as always-available — otherwise HA would mark them
+    # ``unavailable`` forever even though their state is being published.
+    if entity.device.provides_availability:
+        config["availability"] = [{"topic": avail_topic}]
+        config["availability_mode"] = "all"
+
     # Component-specific fields
     if entity.component == "binary_sensor":
         # "online" binary sensor: payload ON/OFF