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/integrations/expose.py

@@ -58,6 +58,21 @@ class DeviceInfo:
     name: str
     """Human-readable device name (may change; triggers HA discovery re-publish)."""
 
+    provides_availability: bool = True
+    """Whether this device publishes an availability ("online"/"offline") heartbeat.
+
+    When True (the default — e.g. Modbus devices, which have an ``online``
+    binary_sensor driving availability), the HA Discovery config for each entity
+    declares an ``availability`` topic and HA marks the entity unavailable until
+    "online" is published.
+
+    When False (e.g. the energy-cost device, which has only sensors and no
+    heartbeat source), the config omits ``availability`` so HA treats the entity
+    as *always available* and shows its state as soon as one arrives.  Without
+    this, such entities would stay perpetually ``unavailable`` in HA even though
+    their state is being published.
+    """
+
 
 @dataclass
 class ExposableEntity:
@@ -422,9 +437,13 @@ def _energy_cost_provider(session: Session) -> list[ExposableEntity]:
         currency = latest_period.currency
 
     # --- Shared DeviceInfo (2-element identifiers — required by ha_discovery.py [1] access) ---
+    # provides_availability=False: the energy-cost device has only sensors and no
+    # online/offline heartbeat, so its entities must be "always available" in HA.
+    # (Otherwise HA shows them unavailable despite state being published.)
     device_info = DeviceInfo(
         identifiers=("energy-cost", "energy-cost"),
         name="Energy Cost",
+        provides_availability=False,
     )
 
     # --- value_getter: current buy price ---