fix: separate onException/errorHandler into distinct RouteFlow segments

ON_EXCEPTION and ERROR_HANDLER nodes are now treated as compound containers
in the ELK diagram renderer, nesting their children. The frontend
diagram-mapping builds separate FlowSegments for each error handler,
displayed as distinct sections in the RouteFlow component.

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

@@ -99,7 +99,7 @@ public class ElkDiagramRenderer implements DiagramRenderer {
     private static final Set<NodeType> COMPOUND_TYPES = EnumSet.of(
             NodeType.EIP_CHOICE, NodeType.EIP_SPLIT, NodeType.TRY_CATCH,
             NodeType.DO_TRY, NodeType.EIP_LOOP, NodeType.EIP_MULTICAST,
-            NodeType.EIP_AGGREGATE
+            NodeType.EIP_AGGREGATE, NodeType.ON_EXCEPTION, NodeType.ERROR_HANDLER
     );
 
     public ElkDiagramRenderer() {