refactor(diagram): 布局计算抽 DiagramLayoutService，PNG 出图不变

src/test/java/com/yunhai/flowcenter/service/DiagramImageServiceTest.java

@@ -18,7 +18,7 @@ import static org.junit.jupiter.api.Assertions.assertTrue;
  */
 class DiagramImageServiceTest {
 
-    private final DiagramImageService service = new DiagramImageService();
+    private final DiagramImageService service = new DiagramImageService(new DiagramLayoutService());
 
     private FlowModels.NodeVo node(String id, String type, String label, String sub, String state) {
         FlowModels.NodeVo n = new FlowModels.NodeVo();