feat: add AI grading test endpoint and update connection logic

frontend/src/components/AdminDashboard.tsx

@@ -135,7 +135,12 @@ export default function AdminDashboard({ onExit }: Readonly<Props>) {
     setTestAiSending(true);
     setTestAiMsg(null);
     try {
-      await adminApi.testAiConnection();
+      await adminApi.testAiConnection({
+        provider: settingsDraft.ai_provider,
+        apiKey: settingsDraft.ai_api_key,
+        model: settingsDraft.ai_model || 'gpt-4o-mini',
+        baseURL: settingsDraft.ai_base_url || undefined,
+      });
       setTestAiMsg({ ok: true, text: t("admin.aiGrading.testSuccess") });
     } catch (e) {
       setTestAiMsg({ ok: false, text: (e as Error).message });