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>

ui/src/api/queries/diagrams.ts

@@ -1,10 +1,21 @@
 import { useQuery } from '@tanstack/react-query';
 import { api } from '../client';
 
+export interface DiagramNode {
+  id?: string;
+  label?: string;
+  type?: string;
+  x?: number;
+  y?: number;
+  width?: number;
+  height?: number;
+  children?: DiagramNode[];
+}
+
 interface DiagramLayout {
   width?: number;
   height?: number;
-  nodes?: Array<{ id?: string; label?: string; type?: string; x?: number; y?: number; width?: number; height?: number }>;
+  nodes?: DiagramNode[];
   edges?: Array<{ from?: string; to?: string }>;
 }