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/Routes/RouteDetail.tsx

@@ -32,7 +32,7 @@ import { useStatsTimeseries, useSearchExecutions, useExecutionStats } from '../.
 import { useApplicationConfig, useUpdateApplicationConfig, useTestExpression } from '../../api/queries/commands';
 import type { TapDefinition } from '../../api/queries/commands';
 import type { ExecutionSummary, AppCatalogEntry, RouteSummary } from '../../api/types';
-import { mapDiagramToRouteNodes, toFlowSegments } from '../../utils/diagram-mapping';
+import { buildFlowSegments } from '../../utils/diagram-mapping';
 import styles from './RouteDetail.module.css';
 
 // ── Row types ────────────────────────────────────────────────────────────────
@@ -366,7 +366,7 @@ export default function RouteDetail() {
   // Route flow from diagram
   const diagramFlows = useMemo(() => {
     if (!diagram?.nodes) return [];
-    return toFlowSegments(mapDiagramToRouteNodes(diagram.nodes, [])).flows;
+    return buildFlowSegments(diagram.nodes, []).flows;
   }, [diagram]);
 
   // Processor table rows