Fix Phase 2b edge case: mostRecentPacketTime when all nodes have lastHeard=0

- Add takeIf check to distinguish lastHeard=0 (never heard) from no nodes
- Previously: maxOfOrNull returns 0, Elvis operator doesn't trigger (0 is not null)
- Now: takeIf { it > 0 } filters out zero, falling back to current time
- Ensures API returns meaningful timestamp instead of epoch 1970

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

core/data/src/commonMain/kotlin/org/meshtastic/core/data/ai/AiFunctionProviderImpl.kt

@@ -260,7 +260,10 @@ class AiFunctionProviderImpl(
 
             // Find most recent packet: max lastHeard across all nodes (convert seconds to ms)
             val mostRecentPacketTimeMs =
-                nodeMap.values.maxOfOrNull { it.lastHeard }?.toLong()?.times(MS_PER_SEC)
+                nodeMap.values.maxOfOrNull { it.lastHeard }
+                    ?.takeIf { it > 0 }
+                    ?.toLong()
+                    ?.times(MS_PER_SEC)
                     ?: clock.now().toEpochMilliseconds()
 
             // Get local device uptime from its DeviceMetrics (node #0 is typically the local device)