Adjust error messages (#1969)

<img width="1378" height="285" alt="image"
src="https://github.com/user-attachments/assets/f0283378-f839-4a1f-8331-909694a04b1f"
/>

codex-rs/login/src/token_data.rs

@@ -67,6 +67,13 @@ impl PlanType {
             }
         }
     }
+
+    pub fn as_string(&self) -> String {
+        match self {
+            Self::Known(known) => format!("{known:?}").to_lowercase(),
+            Self::Unknown(s) => s.clone(),
+        }
+    }
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]