feat(mqtt): add neighbors JSON payload builder (host-tested)

Add buildNeighborsMessage to the pure MQTTPayloadBuilder core and a thin
delegating wrapper + NeighborsMessageEntry alias on MQTTMessageBuilder, so
the neighbors topic is built by the same firmware-facing API as status/
packet/raw while the layout logic stays exercisable by native tests.

The document is bounded to the publish buffer: entries arrive ordered most-
to least-useful and the tail is dropped once the next entry would overflow,
so a fixed PSRAM buffer can never be handed truncated JSON.

Uses ArduinoJson v7 idioms (.to<JsonObject>()/.add<JsonObject>()) to stay
warning-clean under -Werror, unlike the deprecated createNested* forms.

Adds three test_mqtt_payload_builder cases: self+entry round-trip, empty
table / null scopes, and bounded-growth tail-drop under a tight buffer.

src/helpers/MQTTMessageBuilder.cpp

@@ -102,6 +102,22 @@ int MQTTMessageBuilder::buildRawMessage(
       origin, origin_id, timestamp, raw, buffer, buffer_size);
 }
 
+int MQTTMessageBuilder::buildNeighborsMessage(
+  JsonDocument& doc,
+  const char* origin,
+  const char* origin_id,
+  const char* timestamp,
+  const char* self_scopes,
+  const NeighborsMessageEntry* neighbors,
+  int neighbor_count,
+  char* buffer,
+  size_t buffer_size
+) {
+  return MQTTPayloadBuilder::buildNeighborsMessage(
+      doc, origin, origin_id, timestamp, self_scopes, neighbors, neighbor_count,
+      buffer, buffer_size);
+}
+
 int MQTTMessageBuilder::buildPacketJSON(
   JsonDocument& doc,
   mesh::Packet* packet,