fix: 修复ProxiedAirwallexClient的SDK API模块加载

- 设置__module__为airwallex.client，让SDK的__getattr__正确加载api模块
- 修复issuing_cardholder等API属性找不到的问题

backend/app/proxy_client.py

@@ -18,6 +18,9 @@ class ProxiedAirwallexClient(AirwallexClient):
     Uses stdlib http.client for auth to bypass Cloudflare fingerprinting.
     """
 
+    # Override __module__ so SDK's __getattr__ finds api modules under 'airwallex' package
+    __module__ = "airwallex.client"
+
     def __init__(self, proxy_url: str | None = None, login_as: str | None = None, **kwargs):
         self._proxy_url = proxy_url
         self._login_as = login_as