[nexus] clear last parent ID on router/leader role change (#12986)

In `Nexus::Core::HandleStateChanged`, the `lastParentId` was not
cleared when a device transitioned to the Router or Leader role. This
could cause stale parent associations to persist, leading to incorrect
link state reporting in the visualizer during subsequent role
transitions (e.g., when a former Leader merges into a partition and
becomes a Child/REED).

This fix clears `lastParentId` (sets it to `0xffff`) when the device
becomes a Router or Leader, ensuring a fresh state for parent
tracking.

tests/nexus/platform/nexus_core.cpp

@@ -594,6 +594,11 @@ void Core::HandleStateChanged(otChangedFlags aFlags, void *aContext)
         }
         break;
     }
+    case Mle::kRoleRouter:
+    case Mle::kRoleLeader:
+        node->SetLastParentId(0xffff);
+        break;
+
     default:
         break;
     }