perf: add fast-path comparison in MultiResponseMessages (open-webui#22100)

Same optimization as ResponseMessage: add O(1) fast-path check on content and done fields before falling back to full JSON.stringify comparison. Avoids expensive serialization when only content changes during streaming.

Author: Classic298

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

@@ -62,8 +62,13 @@
 
 	let message = structuredClone(history.messages[messageId]);
 	$: if (history.messages) {
-		if (JSON.stringify(message) !== JSON.stringify(history.messages[messageId])) {
-			message = structuredClone(history.messages[messageId]);
+		const source = history.messages[messageId];
+		if (source) {
+			if (message.content !== source.content || message.done !== source.done) {
+				message = structuredClone(source);
+			} else if (JSON.stringify(message) !== JSON.stringify(source)) {
+				message = structuredClone(source);
+			}
 		}
 	}