fix: use raw timestamp string for throughput/error chart data

Avoids Date round-trip that crashes with toISOString() on invalid
timestamps from the timeseries API.

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

ui/src/pages/AgentInstance/AgentInstance.tsx

@@ -80,7 +80,7 @@ export default function AgentInstance() {
         ? (new Date(buckets[1].timestamp).getTime() - new Date(buckets[0].timestamp).getTime()) / 1000
         : 60;
     return buckets.map((b: any) => ({
-      date: new Date(b.timestamp),
+      time: b.timestamp,
       throughput: bucketSecs > 0 ? b.totalCount / bucketSecs : b.totalCount,
       latency: b.avgDurationMs,
       errorPct: b.totalCount > 0 ? (b.failedCount / b.totalCount) * 100 : 0,
@@ -129,12 +129,12 @@ export default function AgentInstance() {
 
   const throughputData = useMemo(() => {
     if (!chartData.length) return [];
-    return chartData.map((d: any) => ({ time: d.date.toISOString(), throughput: d.throughput }));
+    return chartData.map((d: any) => ({ time: d.time, throughput: d.throughput }));
   }, [chartData]);
 
   const errorData = useMemo(() => {
     if (!chartData.length) return [];
-    return chartData.map((d: any) => ({ time: d.date.toISOString(), errorPct: d.errorPct }));
+    return chartData.map((d: any) => ({ time: d.time, errorPct: d.errorPct }));
   }, [chartData]);
 
   // Application logs