ha_discovery: publish state/availability under home_automation prefix

Follow the standard MQTT Discovery split: discovery config stays under the HA
discovery prefix (homeassistant/.../config, required), but the actual state and
availability are published under our own prefix (default home_automation/). The
config payload's state_topic/availability fields point at the new prefix so HA
reads state from there.

- new config ha_state_topic_prefix (default home_automation).
- build_discovery_payload + all publish_* split discovery_prefix (config topic)
  from state_prefix (state/availability). energy-cost still omits availability;
  modbus availability structure unchanged. Tests updated.

tests/test_energy_expose.py

@@ -90,11 +90,13 @@ def _make_settings(
     mqtt_enabled: bool = True,
     ha_discovery_enabled: bool = True,
     ha_discovery_prefix: str = "homeassistant",
+    ha_state_topic_prefix: str = "home_automation",
 ) -> MagicMock:
     s = MagicMock()
     s.mqtt_enabled = mqtt_enabled
     s.ha_discovery_enabled = ha_discovery_enabled
     s.ha_discovery_prefix = ha_discovery_prefix
+    s.ha_state_topic_prefix = ha_state_topic_prefix
     return s
 
 
@@ -691,7 +693,7 @@ def test_energy_entity_discovery_topics_contain_correct_node_id() -> None:
         state_class="total",
     )
 
-    topic, config = build_discovery_payload(entity, prefix="homeassistant")
+    topic, config = build_discovery_payload(entity, discovery_prefix="homeassistant")
 
     # node_id: hyphens replaced with underscores → "energy_cost"
     expected_node = "energy_cost"