SimpleChat:MCUI: Delay enabling user-input to avoid race

Re-enable user-input, only after response to a user query has been
updated to the chat-div. This ensures that if user tries to switch
chat session, it wont be allowed till chat-request-response flow is
done.

examples/server/public_simplechat/simplechat.js

@@ -265,8 +265,6 @@ class MultiChatUI {
             body: theBody,
         });
 
-        this.elInUser.value = "";
-        this.elInUser.disabled = false;
         let respBody = await resp.json();
         console.debug(`DBUG:MCUI:${chatId}:HandleUserSubmit:RespBody:${JSON.stringify(respBody)}`);
         let assistantMsg;
@@ -288,6 +286,8 @@ class MultiChatUI {
         if ((apiEP == ApiEP.Completion) && (gbCompletionFreshChatAlways)) {
             chat.xchat.length = 0;
         }
+        this.elInUser.value = "";
+        this.elInUser.disabled = false;
         this.elInUser.focus();
     }