perf(go): skip empty realtime payload snapshots

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

@@ -321,7 +321,7 @@ func hasRealtimePayload(env envelope.FrameEnvelope) bool {
 	if len(env.Fields) > 0 {
 		return true
 	}
-	if _, ok := env.Parsed["data_units"]; ok {
+	if units, ok := env.Parsed["data_units"].([]any); ok && len(units) > 0 {
 		return true
 	}
 	return false

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

@@ -404,6 +404,25 @@ func TestSnapshotWriterSkipsGB32960HeartbeatWithoutRealtimePayload(t *testing.T)
 	}
 }
 
+func TestSnapshotWriterSkipsEmptyDataUnitsWithoutCoreFields(t *testing.T) {
+	exec := &recordingSnapshotExec{}
+	writer := NewSnapshotWriter(exec)
+
+	if err := writer.Update(context.Background(), envelope.FrameEnvelope{
+		Protocol:     envelope.ProtocolGB32960,
+		MessageID:    "0x02",
+		VIN:          "VIN001",
+		EventTimeMS:  1782918600000,
+		ReceivedAtMS: 1782918601000,
+		Parsed:       map[string]any{"data_units": []any{}},
+	}); err != nil {
+		t.Fatalf("Update() error = %v", err)
+	}
+	if len(exec.calls) != 0 {
+		t.Fatalf("exec calls = %d, want 0", len(exec.calls))
+	}
+}
+
 type recordingSnapshotExec struct {
 	calls []snapshotExecCall
 }