Report device unavailable state through Emulated Hue (#29029)

If an entity is in unavailable state in HA, we expose this
as unreachable to Hue clients. This helps when troubleshooting
Hue issues because you can now receive feedback if there is an issue
on the HA side.

homeassistant/components/emulated_hue/hue_api.py

@@ -56,6 +56,7 @@ from homeassistant.const import (
     SERVICE_VOLUME_SET,
     STATE_OFF,
     STATE_ON,
+    STATE_UNAVAILABLE,
 )
 from homeassistant.util.network import is_local
 
@@ -578,7 +579,7 @@ def entity_to_json(config, entity, state):
                 HUE_API_STATE_BRI: state[STATE_BRIGHTNESS],
                 HUE_API_STATE_HUE: state[STATE_HUE],
                 HUE_API_STATE_SAT: state[STATE_SATURATION],
-                "reachable": True,
+                "reachable": entity.state != STATE_UNAVAILABLE,
             },
             "type": "Dimmable light",
             "name": config.get_entity_name(entity),
@@ -587,7 +588,10 @@ def entity_to_json(config, entity, state):
             "swversion": "123",
         }
     return {
-        "state": {HUE_API_STATE_ON: state[STATE_ON], "reachable": True},
+        "state": {
+            HUE_API_STATE_ON: state[STATE_ON],
+            "reachable": entity.state != STATE_UNAVAILABLE,
+        },
         "type": "On/off light",
         "name": config.get_entity_name(entity),
         "modelid": "HASS321",