Support ppb and ppm for VOC reporting in Home Assistant

See home-assistant/core#92973.

lib/extension/homeassistant.ts

@@ -908,10 +908,15 @@ export default class HomeAssistant extends Extension {
                 delete discoveryEntry.discovery_payload.device_class;
             }
 
-            // Home Assistant only supports µg/m³, not other units like ppb.
+            // Home Assistant uses a different voc device_class for µg/m³ versus ppb or ppm.
             // https://github.com/Koenkk/zigbee2mqtt/issues/16057
             if (firstExpose.name === 'voc' && discoveryEntry.discovery_payload.unit_of_measurement !== 'µg/m³') {
-                delete discoveryEntry.discovery_payload.device_class;
+                if (discoveryEntry.discovery_payload.unit_of_measurement === 'ppb' ||
+                    discoveryEntry.discovery_payload.unit_of_measurement === 'ppm') {
+                    discoveryEntry.discovery_payload.device_class = 'volatile_organic_compounds_parts';
+                } else {
+                    delete discoveryEntry.discovery_payload.device_class;
+                }
             }
 
             // entity_category config is not allowed for sensors