fix(ui): give rx_snr real presence semantics end to end (#6523)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

androidApp/src/fdroid/kotlin/org/meshtastic/app/map/discovery/DiscoveryOsmMap.kt

@@ -122,7 +122,7 @@ fun DiscoveryOsmMap(
                         position = nodeGeoPoint
                         setAnchor(Marker.ANCHOR_CENTER, Marker.ANCHOR_BOTTOM)
                         title = node.longName ?: node.shortName ?: "Unknown"
-                        snippet = "SNR: ${node.snr} dB / RSSI: ${MetricFormatter.rssi(node.rssi)}"
+                        snippet = "SNR: ${MetricFormatter.snr(node.snr)} / RSSI: ${MetricFormatter.rssi(node.rssi)}"
 
                         val drawableId =
                             if (node.isSensorNode) {

androidApp/src/google/kotlin/org/meshtastic/app/ai/appfunctions/AppFunctionModels.kt

@@ -126,10 +126,10 @@ data class GetNodeDetailsResponse(
     val hardwareModel: String,
     /** Firmware version string. */
     val firmwareVersion: String,
-    /** Signal-to-noise ratio of strongest signal. */
-    val snr: Float,
-    /** Received signal strength indicator in dB. */
-    val rssi: Int,
+    /** Signal-to-noise ratio in dB of the strongest signal, or null if this node has no reading. */
+    val snr: Float?,
+    /** Received signal strength indicator in dBm, or null if this node has no reading. */
+    val rssi: Int?,
     /** Number of hops away from local node (-1 if unknown). */
     val hopsAway: Int,
     /** Channel index this node is on. */

androidApp/src/google/kotlin/org/meshtastic/app/map/discovery/DiscoveryGoogleMap.kt

@@ -126,7 +126,7 @@ fun DiscoveryGoogleMap(
             MarkerComposable(
                 state = rememberUpdatedMarkerState(position = nodeLatLng),
                 title = node.longName ?: node.shortName ?: "Unknown",
-                snippet = "SNR: ${node.snr} dB / RSSI: ${MetricFormatter.rssi(node.rssi)}",
+                snippet = "SNR: ${MetricFormatter.snr(node.snr)} / RSSI: ${MetricFormatter.rssi(node.rssi)}",
             ) {
                 DiscoveryMarkerChip(label = node.shortName ?: "?", color = markerColor, icon = nodeIcon)
             }