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.
agessaman
e36aee04d4