Add Enterprise plan to ChatGPT login description (openai#6918)

## Summary
- update ChatGPT onboarding login description to mention Enterprise
plans alongside Plus, Pro, and Team

## Testing
- just fmt


------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_691e088daf20832c88d8b667adf45128)

Author: ae-openai

codex-rs/tui/src/onboarding/auth.rs

@@ -217,7 +217,7 @@ impl AuthModeWidget {
         };
 
         let chatgpt_description = if self.is_chatgpt_login_allowed() {
-            "Usage included with Plus, Pro, and Team plans"
+            "Usage included with Plus, Pro, Team, and Enterprise plans"
         } else {
             "ChatGPT login is disabled"
         };