Add on_new_device support and parent device via_device linking

Register devices in HA device registry via on_new_device callback,
which fires before on_new_metric in topological order. Sub-devices
(like SwitchableOutputs) are linked to their parent via via_device.

Changes:
- _on_new_device: registers devices in HA device registry proactively
- _map_device_info: uses device.parent_device for via_device instead
  of hardcoding system_0
- DeviceType check for system device instead of unique_id comparison

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: tomer-w

custom_components/victron_mqtt/hub.py

@@ -24,6 +24,7 @@
 )
 from homeassistant.core import HomeAssistant
 from homeassistant.exceptions import ConfigEntryAuthFailed, ConfigEntryNotReady
+from homeassistant.helpers import device_registry as dr
 from homeassistant.helpers.device_registry import DeviceInfo
 from homeassistant.helpers.redact import async_redact_data
 
@@ -106,6 +107,8 @@ def __init__(self, hass: HomeAssistant, entry: VictronGxConfigEntry) -> None:
             ),
         )
         self._hub.on_new_metric = self._on_new_metric
+        self._hub.on_new_device = self._on_new_device
+        self._config_entry_id = entry.entry_id
         self.new_metric_callbacks: dict[MetricKind, NewMetricCallback] = {}
 
     async def start(self) -> None:
@@ -127,6 +130,21 @@ async def stop(self) -> None:
         _LOGGER.info("Stopping hub")
         await self._hub.disconnect()
 
+    def _on_new_device(
+        self,
+        hub: VictronVenusHub,
+        device: VictronVenusDevice,
+    ) -> None:
+        """Register a new device in the HA device registry."""
+        _LOGGER.info("New device received. Device: %s", device)
+        assert hub.installation_id is not None
+        device_info = Hub._map_device_info(device, hub.installation_id)
+        device_registry = dr.async_get(self.hass)
+        device_registry.async_get_or_create(
+            config_entry_id=self._config_entry_id,
+            **device_info,
+        )
+
     def _on_new_metric(
         self,
         hub: VictronVenusHub,
@@ -155,8 +173,10 @@ def _map_device_info(
             model=device.model,
             serial_number=device.serial_number,
         )
-        # Don't set via_device for the GX device itself
-        if device.unique_id != "system_0":
+        # Set via_device based on parent_device relationship
+        if device.parent_device is not None:
+            device_info["via_device"] = (DOMAIN, f"{installation_id}_{device.parent_device.unique_id}")
+        elif device.device_type != DeviceType.SYSTEM:
             device_info["via_device"] = (DOMAIN, f"{installation_id}_system_0")
         return device_info