feat(ui): add endpoint format hint for Codex providers

Add informational hint below Codex API endpoint input field to guide users
to fill in OpenAI Response compatible service endpoints, similar to the
existing hint for Claude providers.

src/components/ProviderForm.tsx

@@ -1774,6 +1774,11 @@ const ProviderForm: React.FC<ProviderFormProps> = ({
                   autoComplete="off"
                   className="w-full px-3 py-2 border border-gray-200 dark:border-gray-700 dark:bg-gray-800 dark:text-gray-100 rounded-lg text-sm focus:outline-none focus:ring-2 focus:ring-blue-500/20 dark:focus:ring-blue-400/20 focus:border-blue-500 dark:focus:border-blue-400 transition-colors"
                 />
+                <div className="p-3 bg-amber-50 dark:bg-amber-900/20 border border-amber-200 dark:border-amber-700 rounded-lg">
+                  <p className="text-xs text-amber-600 dark:text-amber-400">
+                    {t("providerForm.codexApiHint")}
+                  </p>
+                </div>
               </div>
             )}