feat(go): reuse parsed fields across projections

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

@@ -257,6 +257,57 @@ func TestSnapshotWriterMergesGB32960ParsedJSONAcrossSplitRealtimeFrames(t *testi
 	}
 }
 
+func TestSnapshotWriterUsesEnvelopeParsedFieldsWithoutReflattening(t *testing.T) {
+	db, mock, err := sqlmock.New()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+	writer := NewSnapshotWriter(db)
+
+	mock.ExpectQuery("SELECT parsed_json FROM vehicle_realtime_snapshot WHERE protocol = \\? AND vin = \\?").
+		WithArgs("GB32960", "VIN001").
+		WillReturnRows(sqlmock.NewRows([]string{"parsed_json"}))
+	mock.ExpectExec("INSERT INTO vehicle_realtime_snapshot").
+		WithArgs(
+			"GB32960",
+			"VIN001",
+			"",
+			"",
+			"",
+			jsonFlatFieldsArg{fields: map[string]string{
+				"gb32960.vehicle.speed_kmh": "12.3",
+			}},
+			sqlmock.AnyArg(),
+			sqlmock.AnyArg(),
+			sqlmock.AnyArg(),
+		).
+		WillReturnResult(sqlmock.NewResult(0, 1))
+
+	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{
+				map[string]any{"type": "0x01", "name": "vehicle", "value": map[string]any{"speed_kmh": 99}},
+			},
+		},
+		ParsedFields: map[string]any{
+			"gb32960.vehicle.speed_kmh": "12.3",
+		},
+		Fields: map[string]any{envelope.FieldSpeedKMH: 12.3},
+	})
+	if err != nil {
+		t.Fatalf("Update() error = %v", err)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Fatalf("sql expectations: %v", err)
+	}
+}
+
 func TestSnapshotWriterCollapsesGB32960StackFragmentsWhenMergingParsedJSON(t *testing.T) {
 	db, mock, err := sqlmock.New()
 	if err != nil {