test(car): cover the buildNodeUi snr resolver path

Address review feedback on #6523.

The car signal-quality tests called determineSignalQuality() directly, so
nothing proved buildNodeUi() routes through Node.snrOrNull rather than reading
node.snr. Add two buildNodeUi() cases: an unset node resolves to UNKNOWN (this
fails if the call reverts to node.snr, which would feed Float.MAX_VALUE into the
bands and rate a node with no reading as EXCELLENT), and a 0 dB node still rates
EXCELLENT.

Also document the nullable snr/rssi fields in the AppFunction schema and correct
RSSI's unit from dB to dBm to match the NodeDetails contract, and record in
SnrExtensionsTest why the proto-absent case is not asserted there: rx_snr is
still a non-null float upstream, so snrOrNull() cannot return null for any
packet the test could construct. Null handling is covered where a null is
representable, in MetricFormatterTest and LoraSignalIndicatorUiTest.

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

Author: jamesarich

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

@@ -126,9 +126,9 @@ data class GetNodeDetailsResponse(
     val hardwareModel: String,
     /** Firmware version string. */
     val firmwareVersion: String,
-    /** Signal-to-noise ratio of strongest signal. */
+    /** 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 dB. */
+    /** 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,

core/model/src/commonTest/kotlin/org/meshtastic/core/model/util/SnrExtensionsTest.kt

@@ -24,6 +24,13 @@ import kotlin.test.assertNotNull
 /**
  * Guards the presence policy for `rx_snr`: absent means null and nothing else. Unlike [rxTimeOrNull], a zero must never
  * be folded into "unknown" — 0 dB is a real measurement. See [snrOrNull].
+ *
+ * The proto-absent case is not asserted here because it is not yet constructible: `rx_snr` is still a non-null `float`
+ * upstream, so [snrOrNull] cannot return null for any packet this test could build. What these tests do lock down is
+ * the half that can regress today — that a zero is never folded — which is exactly what breaks if the [rxTimeOrNull]
+ * pattern is copied over. Null *handling* is covered where a null is representable: `MetricFormatterTest`
+ * (`snrAbsentIsUnknown`) and `LoraSignalIndicatorUiTest` (`snrRendersNothingWhenAbsent`,
+ * `loraSignalIndicatorShowsUnknownWhenSnrIsAbsent`).
  */
 class SnrExtensionsTest {
 

feature/car/src/test/kotlin/org/meshtastic/feature/car/util/CarScreenDataBuilderTest.kt

@@ -70,6 +70,26 @@ class CarScreenDataBuilderTest {
         assertEquals(SignalQuality.EXCELLENT, quality)
     }
 
+    @Test
+    fun `buildNodeUi resolves an unset node snr to unknown`() {
+        // Exercises the production path: fails if buildNodeUi reverts to reading node.snr, which would feed the
+        // Float.MAX_VALUE sentinel into the bands and rate a node with no reading as EXCELLENT.
+        val node = Node(num = 1)
+
+        val ui = CarScreenDataBuilder.buildNodeUi(node, ModemPreset.LONG_FAST)
+
+        assertEquals(SignalQuality.UNKNOWN, ui.signalQuality)
+    }
+
+    @Test
+    fun `buildNodeUi rates a zero node snr reading`() {
+        val node = Node(num = 1, snr = 0f)
+
+        val ui = CarScreenDataBuilder.buildNodeUi(node, ModemPreset.LONG_FAST)
+
+        assertEquals(SignalQuality.EXCELLENT, ui.signalQuality)
+    }
+
     @Test
     fun `determineSignalQuality returns excellent well above the preset floor`() {
         // LongFast floor -17.5; -10 is 7.5 dB above it (> floor + 5.5 margin).