fix: cross-chat data corruption and tiptap editor access errors (#20266)

* fix: prevent cross-chat corruption and editor access errors

* fix: safety check in Messages.svelte and cleanup in RichTextInput

src/lib/components/chat/Messages.svelte

@@ -346,6 +346,10 @@
 	};
 
 	const saveMessage = async (messageId, message) => {
+		if (!history.messages?.[messageId]) {
+			return;
+		}
+
 		history.messages[messageId] = message;
 		await updateChat();
 	};