feat: support ON_COMPLETION handler sections in diagram

Add ON_COMPLETION to backend COMPOUND_TYPES and frontend rendering.
Completion handlers render as teal-tinted sections between the main
flow and error handlers, structurally parallel to onException.

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

@@ -102,7 +102,8 @@ public class ElkDiagramRenderer implements DiagramRenderer {
             NodeType.EIP_SPLIT, NodeType.TRY_CATCH,
             NodeType.DO_TRY, NodeType.DO_CATCH, NodeType.DO_FINALLY,
             NodeType.EIP_LOOP, NodeType.EIP_MULTICAST,
-            NodeType.EIP_AGGREGATE, NodeType.ON_EXCEPTION, NodeType.ERROR_HANDLER
+            NodeType.EIP_AGGREGATE, NodeType.ON_EXCEPTION, NodeType.ERROR_HANDLER,
+            NodeType.ON_COMPLETION
     );
 
     public ElkDiagramRenderer() {