test: add json stability and discovery payload coverage

lib/dd3_transport_logic/include/ha_discovery_json.h

@@ -0,0 +1,7 @@
+#pragma once
+
+#include <Arduino.h>
+
+bool ha_build_discovery_sensor_payload(const char *device_id, const char *key, const char *name, const char *unit,
+                                       const char *device_class, const char *state_topic, const char *value_template,
+                                       const char *manufacturer, String &out_payload);