feat: add model directive management

apps/aether-gateway/src/tests/control/admin/system.rs

@@ -1224,6 +1224,31 @@ async fn gateway_handles_admin_system_format_conversion_default_as_disabled() {
     upstream_handle.abort();
 }
 
+#[tokio::test]
+async fn gateway_handles_admin_system_model_directives_default_as_disabled() {
+    let gateway = build_router_with_state(AppState::new().expect("gateway should build"));
+    let (gateway_url, gateway_handle) = start_server(gateway).await;
+
+    let response = reqwest::Client::new()
+        .get(format!(
+            "{gateway_url}/api/admin/system/configs/enable_model_directives"
+        ))
+        .header(crate::constants::GATEWAY_HEADER, "rust-phase3b")
+        .header(TRUSTED_ADMIN_USER_ID_HEADER, "admin-user-123")
+        .header(TRUSTED_ADMIN_USER_ROLE_HEADER, "admin")
+        .header(TRUSTED_ADMIN_SESSION_ID_HEADER, "session-123")
+        .send()
+        .await
+        .expect("request should succeed");
+
+    assert_eq!(response.status(), StatusCode::OK);
+    let payload: serde_json::Value = response.json().await.expect("json body should parse");
+    assert_eq!(payload["key"], "enable_model_directives");
+    assert_eq!(payload["value"], json!(false));
+
+    gateway_handle.abort();
+}
+
 #[tokio::test]
 async fn gateway_handles_admin_system_provider_priority_mode_locally_with_bearer_admin_session() {
     let upstream_hits = Arc::new(Mutex::new(0usize));