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/pages/Dashboard/Dashboard.tsx

@@ -21,7 +21,7 @@ import {
 } from '../../api/queries/executions'
 import { useDiagramLayout } from '../../api/queries/diagrams'
 import type { ExecutionSummary } from '../../api/types'
-import { mapDiagramToRouteNodes, toFlowSegments } from '../../utils/diagram-mapping'
+import { buildFlowSegments } from '../../utils/diagram-mapping'
 import styles from './Dashboard.module.css'
 
 // Row type extends ExecutionSummary with an `id` field for DataTable
@@ -376,7 +376,7 @@ export default function Dashboard() {
 
   const routeFlows = useMemo(() => {
     if (diagram?.nodes) {
-      return toFlowSegments(mapDiagramToRouteNodes(diagram.nodes || [], procList)).flows
+      return buildFlowSegments(diagram.nodes || [], procList).flows
     }
     return []
   }, [diagram, procList])