fix: add thread-safe service cache in manager

nepali_payment/manager.py

@@ -1,4 +1,5 @@
 from typing import Any, TypeVar
+import threading
 
 from nepali_payment.enums import PaymentMethod, PaymentMode
 from nepali_payment.exceptions import ValidationError
@@ -45,6 +46,7 @@ class PaymentManager:
         self._api = api
         self._config = config
         self._service_cache = None
+        self._lock = threading.Lock()
 
     def _api_service(self):
         """Return the shared HTTP client, creating it on the first call.
@@ -65,6 +67,8 @@ class PaymentManager:
         ConnectIPS it avoids re-parsing the RSA certificate on every call) and
         keeps a stable HTTP client across operations.
         """
+        if self._service_cache is None:
+            with self._lock:
                 if self._service_cache is None:
                     self._service_cache = get_payment_service(
                         self._method, self._secret_key, self._mode, api=self._api_service(), config=self._config