refactor(opcuaclient): remove vestigial TargetNamespaceKind + UnsMappingTable (#648)

Nothing read the mapping table beyond the startup validator; v3 UNS projection is
explicit via UnsTagReference. Legacy keys in stored DriverConfig are ignored (pinned).
Reverts the docker-dev opc-plc seed workaround from fdee129f.

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientFailoverTests.cs

@@ -72,9 +72,6 @@ public sealed class OpcUaClientFailoverTests
             PerEndpointConnectTimeout = TimeSpan.FromMilliseconds(500),
             Timeout = TimeSpan.FromMilliseconds(500),
             AutoAcceptCertificates = true,
-            // SystemPlatform kind needs no UNS mapping table — keeps this failover test
-            // focused on the endpoint sweep rather than §8 namespace validation.
-            TargetNamespaceKind = OpcUaTargetNamespaceKind.SystemPlatform,
         };
         using var drv = new OpcUaClientDriver(opts, "opcua-failover");