feat: add OpenSearch highlight snippets to search results

- Add highlight field to ExecutionSummary record
- Request highlight fragments from OpenSearch when full-text search is active
- Pass matchContext to command palette for display

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

ui/src/components/LayoutShell.tsx

@@ -137,6 +137,7 @@ function LayoutContent() {
       meta: `${e.routeId} · ${e.applicationName ?? ''} · ${formatDuration(e.durationMs)}`,
       path: `/exchanges/${e.executionId}`,
       serverFiltered: true,
+      matchContext: e.highlight ?? undefined,
     }));
     return [...catalogData, ...exchangeItems];
   }, [catalogData, exchangeResults]);