feat(utils): add message merging utility for persona responses

src/lib/utils/mergeMessages.ts

@@ -0,0 +1,43 @@
+import type { Message } from "$lib/types/Message";
+
+/**
+ * Merges server messages with local optimistic messages to preserve streaming content
+ * while updating structure/IDs from the server.
+ */
+export function mergeMessages(serverMessages: Message[], localMessages: Message[]): Message[] {
+	if (serverMessages.length === 0) return localMessages;
+	if (localMessages.length === 0) return serverMessages;
+
+	const lastServerMsg = serverMessages[serverMessages.length - 1];
+	const lastLocalMsg = localMessages[localMessages.length - 1];
+
+	// Check if they look like the same message (same role)
+	if (lastServerMsg.from === lastLocalMsg.from) {
+		// If server content is empty/short but local is long, keep local content
+		if ((lastServerMsg.content?.length || 0) < (lastLocalMsg.content?.length || 0)) {
+			const newLastServerMsg = { ...lastServerMsg };
+
+			// Adopt server ID but keep local content
+			newLastServerMsg.content = lastLocalMsg.content;
+			newLastServerMsg.reasoning = lastLocalMsg.reasoning;
+
+			// Merge persona responses
+			if (lastLocalMsg.personaResponses?.length && !lastServerMsg.personaResponses?.length) {
+				newLastServerMsg.personaResponses = lastLocalMsg.personaResponses;
+			} else if (lastLocalMsg.personaResponses && lastServerMsg.personaResponses) {
+				newLastServerMsg.personaResponses = lastServerMsg.personaResponses.map((sPR) => {
+					const lPR = lastLocalMsg.personaResponses?.find((l) => l.personaId === sPR.personaId);
+					if (lPR && lPR.content.length > sPR.content.length) {
+						return { ...sPR, content: lPR.content };
+					}
+					return sPR;
+				});
+			}
+
+			// Return new array with replaced last message
+			return [...serverMessages.slice(0, -1), newLastServerMsg];
+		}
+	}
+
+	return serverMessages;
+}