fix: keep valid realtime mileage

go/vehicle-gateway/internal/realtime/repository_test.go

@@ -94,6 +94,47 @@ func TestRepositoryOnlineStatus(t *testing.T) {
 	}
 }
 
+func TestRepositoryDoesNotOverwritePositiveMileageWithZero(t *testing.T) {
+	repo, closeFn := newTestRepository(t)
+	defer closeFn()
+	ctx := context.Background()
+
+	if err := repo.Update(ctx, envelope.FrameEnvelope{
+		Protocol:     envelope.ProtocolJT808,
+		VIN:          "VIN001",
+		EventTimeMS:  1000,
+		ReceivedAtMS: 1100,
+		Fields: map[string]any{
+			envelope.FieldTotalMileageKM: 12345.6,
+		},
+	}); err != nil {
+		t.Fatalf("Update() error = %v", err)
+	}
+	if err := repo.Update(ctx, envelope.FrameEnvelope{
+		Protocol:     envelope.ProtocolJT808,
+		VIN:          "VIN001",
+		EventTimeMS:  2000,
+		ReceivedAtMS: 2100,
+		Fields: map[string]any{
+			envelope.FieldSpeedKMH:       22.0,
+			envelope.FieldTotalMileageKM: 0,
+		},
+	}); err != nil {
+		t.Fatalf("Update() error = %v", err)
+	}
+
+	merged, err := repo.GetMerged(ctx, "VIN001")
+	if err != nil {
+		t.Fatalf("GetMerged() error = %v", err)
+	}
+	if merged.Fields[envelope.FieldTotalMileageKM] != 12345.6 {
+		t.Fatalf("zero mileage overwrote positive value: %#v", merged.Fields)
+	}
+	if merged.Fields[envelope.FieldSpeedKMH] != 22.0 {
+		t.Fatalf("new speed should still merge: %#v", merged.Fields)
+	}
+}
+
 func TestHandlerReturnsMergedSnapshot(t *testing.T) {
 	repo, closeFn := newTestRepository(t)
 	defer closeFn()