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);

lib/dd3_transport_logic/src/ha_discovery_json.cpp

@@ -0,0 +1,37 @@
+#include "ha_discovery_json.h"
+
+#include <ArduinoJson.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) {
+  if (!device_id || !key || !name || !state_topic || !value_template || !manufacturer) {
+    return false;
+  }
+
+  StaticJsonDocument<256> doc;
+  String unique_id = String(device_id) + "_" + key;
+  String sensor_name = String(device_id) + " " + name;
+
+  doc["name"] = sensor_name;
+  doc["state_topic"] = state_topic;
+  doc["unique_id"] = unique_id;
+  if (unit && unit[0] != '\0') {
+    doc["unit_of_measurement"] = unit;
+  }
+  if (device_class && device_class[0] != '\0') {
+    doc["device_class"] = device_class;
+  }
+  doc["value_template"] = value_template;
+
+  JsonObject device = doc.createNestedObject("device");
+  JsonArray identifiers = device.createNestedArray("identifiers");
+  identifiers.add(String(device_id));
+  device["name"] = String(device_id);
+  device["model"] = "DD3-LoRa-Bridge";
+  device["manufacturer"] = manufacturer;
+
+  out_payload = "";
+  size_t len = serializeJson(doc, out_payload);
+  return len > 0;
+}