perf: fast-path length check in StatusHistory comparison (#22103)

Add O(1) array length check before expensive JSON.stringify comparison. During streaming, status history typically only grows via appends, so a length mismatch catches most updates without serialization.

src/lib/components/chat/Messages/ResponseMessage/StatusHistory.svelte

@@ -21,7 +21,8 @@
 		status = history.at(-1);
 	}
 
-	$: if (JSON.stringify(statusHistory) !== JSON.stringify(history)) {
+	$: if (statusHistory.length !== history.length
+		|| JSON.stringify(statusHistory) !== JSON.stringify(history)) {
 		history = statusHistory;
 	}
 </script>