perf: use structuredClone for message deep copies (#21948)

src/lib/components/chat/Messages/MultiResponseMessages.svelte

@@ -60,10 +60,10 @@
 
 	let selectedModelIdx = null;
 
-	let message = JSON.parse(JSON.stringify(history.messages[messageId]));
+	let message = structuredClone(history.messages[messageId]);
 	$: if (history.messages) {
 		if (JSON.stringify(message) !== JSON.stringify(history.messages[messageId])) {
-			message = JSON.parse(JSON.stringify(history.messages[messageId]));
+			message = structuredClone(history.messages[messageId]);
 		}
 	}
 

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

@@ -119,17 +119,17 @@
 	export let messageId;
 	export let selectedModels = [];
 
-	let message: MessageType = JSON.parse(JSON.stringify(history.messages[messageId]));
+	let message: MessageType = structuredClone(history.messages[messageId]);
 	$: if (history.messages) {
 		const source = history.messages[messageId];
 		if (source) {
 			// Fast path: O(1) check on the fields that change most often (content during streaming, done at end)
 			// Avoids 2x O(n) JSON.stringify calls that are always true during streaming anyway
 			if (message.content !== source.content || message.done !== source.done) {
-				message = JSON.parse(JSON.stringify(source));
+				message = structuredClone(source);
 			} else if (JSON.stringify(message) !== JSON.stringify(source)) {
 				// Slow path: full comparison for infrequent changes (sources, annotations, status, etc.)
-				message = JSON.parse(JSON.stringify(source));
+				message = structuredClone(source);
 			}
 		}
 	}