fix(mqtt): omit SNR/RSSI instead of publishing placeholder values

The reconstruction-less fallback path in buildPacketJSON (queued packet
with no captured raw frame that cannot be re-serialized) published
fabricated SNR 12.5 / RSSI -65 that consumers could not distinguish
from real radio readings. It now passes NAN / MQTT_RSSI_UNKNOWN, and
buildPacketMessage omits the SNR/RSSI keys entirely when no measurement
exists, mirroring the existing score omission.

Ports the fix from ACETyr's PR #13 (0a037040) on mqtt-bridge-implementation.

Co-Authored-By: ACETyr <35745290+ACETyr@users.noreply.github.com>

src/helpers/MQTTMessageBuilder.cpp

@@ -209,8 +209,8 @@ int MQTTMessageBuilder::buildPacketJSON(
     packet_type, route_str,
     packet->payload_len,
     raw_hex,
-    12.5f, // SNR - using reasonable default
-    -65,   // RSSI - using reasonable default
+    NAN,   // SNR - unknown on this reconstruction-less fallback path
+    MQTT_RSSI_UNKNOWN, // RSSI - unknown on this reconstruction-less fallback path
     NAN,   // score - unknown on this reconstruction-less fallback path
     hash_str,
     has_path ? packet->path : nullptr,

src/helpers/MQTTPayloadBuilder.cpp

@@ -120,8 +120,6 @@ int MQTTPayloadBuilder::buildPacketMessage(
   snprintf(len_str, sizeof(len_str), "%d", len);
   snprintf(packet_type_str, sizeof(packet_type_str), "%d", packet_type);
   snprintf(payload_len_str, sizeof(payload_len_str), "%d", payload_len);
-  snprintf(snr_str, sizeof(snr_str), "%.1f", snr);
-  snprintf(rssi_str, sizeof(rssi_str), "%d", rssi);
 
   root["timestamp"] = timestamp;
   root["hash"] = hash;
@@ -138,8 +136,16 @@ int MQTTPayloadBuilder::buildPacketMessage(
   root["origin_id"] = origin_id;
 
   if (direction && strcmp(direction, "rx") == 0) {
-    root["SNR"] = snr_str;
-    root["RSSI"] = rssi_str;
+    // Omit SNR/RSSI entirely when no measurement exists rather than publish
+    // a placeholder a consumer could mistake for a real reading.
+    if (!isnan(snr)) {
+      snprintf(snr_str, sizeof(snr_str), "%.1f", snr);
+      root["SNR"] = snr_str;
+    }
+    if (rssi != MQTT_RSSI_UNKNOWN) {
+      snprintf(rssi_str, sizeof(rssi_str), "%d", rssi);
+      root["RSSI"] = rssi_str;
+    }
     if (!isnan(score)) {
       snprintf(score_str, sizeof(score_str), "%d", static_cast<int>(score * 1000));
       root["score"] = score_str;

src/helpers/MQTTPayloadBuilder.h

@@ -7,6 +7,11 @@
 // Mesh-independent JSON serialization core for MQTT publication payloads.
 // MQTTMessageBuilder keeps the firmware-facing API and delegates these three
 // deterministic contracts here so they can be exercised by native tests.
+
+// RSSI sentinel for "no radio measurement": far outside the small negative ints
+// a radio actually reports, so a real reading can never collide with it.
+static const int MQTT_RSSI_UNKNOWN = INT16_MIN;
+
 class MQTTPayloadBuilder {
 public:
   static int buildStatusMessage(