fix: show tracing action on all Flow view nodes

Use diagram node ID as fallback processorId when no processor
execution match exists (e.g. error handlers that didn't trigger).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/ExchangeDetail/ExchangeDetail.tsx

@@ -157,7 +157,7 @@ export default function ExchangeDetail() {
     const lookup = new Map(flatProcs
       .filter(p => p.diagramNodeId && p.processorId)
       .map(p => [p.diagramNodeId!, p.processorId!]))
-    return diagram.nodes.map(node => lookup.get(node.id ?? '') ?? '')
+    return diagram.nodes.map(node => lookup.get(node.id ?? '') ?? node.id ?? '')
   }, [diagram, procList, processorIds])
 
   // ── Tracing toggle ──────────────────────────────────────────────────────