fix: skip edges that cross ELK root graph boundaries

Edges connecting main flow nodes to handler section nodes (ON_EXCEPTION,
ON_COMPLETION) now span different ELK root graphs. ELK throws
UnsupportedGraphException when an edge connects nodes in different
layout hierarchies. Skip these cross-root edges — the frontend doesn't
render them anyway (handler sections are separated visually).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-app/src/main/java/com/cameleer3/server/app/diagram/ElkDiagramRenderer.java

@@ -250,7 +250,8 @@ public class ElkDiagramRenderer implements DiagramRenderer {
             handlerRoots.add(handlerRoot);
         }
 
-        // Create ELK edges
+        // Create ELK edges — skip edges that cross between different ELK root graphs
+        // (e.g., main flow → handler section). These cannot be laid out by ELK.
         if (graph.getEdges() != null) {
             for (RouteEdge re : graph.getEdges()) {
                 ElkNode sourceElk = elkNodeMap.get(re.getSource());
@@ -259,6 +260,13 @@ public class ElkDiagramRenderer implements DiagramRenderer {
                     continue;
                 }
 
+                // Skip edges that cross ELK root boundaries
+                ElkNode sourceRoot = getElkRoot(sourceElk);
+                ElkNode targetRoot = getElkRoot(targetElk);
+                if (sourceRoot != targetRoot) {
+                    continue;
+                }
+
                 // Determine the containing node for the edge
                 ElkNode containingNode = findCommonParent(sourceElk, targetElk);
 
@@ -542,6 +550,15 @@ public class ElkDiagramRenderer implements DiagramRenderer {
     // ELK graph helpers
     // ----------------------------------------------------------------
 
+    /** Walk up to the top-level root of an ELK node's hierarchy. */
+    private ElkNode getElkRoot(ElkNode node) {
+        ElkNode current = node;
+        while (current.getParent() != null) {
+            current = current.getParent();
+        }
+        return current;
+    }
+
     private ElkNode findCommonParent(ElkNode a, ElkNode b) {
         if (a.getParent() == b.getParent()) {
             return a.getParent();