fix: Remove explicit security policy setting from OPC UA client

Let the client automatically negotiate security policy with the server instead of trying to set it explicitly

src/dashboard/protocol_clients.py

@@ -23,12 +23,15 @@ class OPCUAClient:
         """Connect to OPC UA server."""
         try:
             from asyncua import Client
+            from asyncua.crypto.security_policies import SecurityPolicyBasic256Sha256
+            
             self._client = Client(url=self.endpoint)
             
             # Explicitly set security policy to match server configuration
             # The server supports both Basic256Sha256 and None security policies
             # Use None for development/testing
-            await self._client.set_security_string("http://opcfoundation.org/UA/SecurityPolicy#None")
+            # Try connecting without explicit security policy first
+            # The client should automatically negotiate with the server
             
             # Set timeout for connection
             await asyncio.wait_for(self._client.connect(), timeout=5.0)