[api] Fix sending the `once` flag on ha entity subscription (#7357)

esphome/components/api/api_connection.cpp

@@ -179,6 +179,7 @@ void APIConnection::loop() {
       SubscribeHomeAssistantStateResponse resp;
       resp.entity_id = it.entity_id;
       resp.attribute = it.attribute.value();
+      resp.once = it.once;
       if (this->send_subscribe_home_assistant_state_response(resp)) {
         state_subs_at_++;
       }