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.
Replace direct JSON construction in MQTTMessageBuilder with calls to
MQTTPayloadBuilder for building status, packet, and raw messages.
This change improves code maintainability and reduces duplication by
centralizing message formatting logic. Additionally, update platformio.ini
to include ArduinoJson dependency for JSON handling.
mikecarper