fix(infra): use appConfig.Validate instead of CheckApplicationInstanceCertificate

Replace with Validate() which validates config without requiring a cert,
matching the RealOpcUaClient pattern. Fixes OPC UA connection failure.

infra/lmxfakeproxy/Bridge/OpcUaBridge.cs

@@ -46,13 +46,7 @@ public class OpcUaBridge : IOpcUaBridge
             TransportQuotas = new TransportQuotas { OperationTimeout = 15000 }
         };
 
-        var application = new ApplicationInstance
-        {
-            ApplicationName = "LmxFakeProxy",
-            ApplicationType = ApplicationType.Client,
-            ApplicationConfiguration = appConfig
-        };
-        await application.CheckApplicationInstanceCertificate(false, 0);
+        await appConfig.Validate(ApplicationType.Client);
         appConfig.CertificateValidator.CertificateValidation += (_, e) => e.Accept = true;
 
         EndpointDescription? endpoint;