feat(opcua,host): F13c LDAP-bound UserName validator

Adds IOpcUaUserAuthenticator seam in OpcUaServer.Security with a deny-all
NullOpcUaUserAuthenticator default. OpcUaApplicationHost subscribes to
SessionManager.ImpersonateUser after _application.Start so UserName tokens
flow through the authenticator and either attach a UserIdentity to the
session (Allow) or set IdentityValidationError = BadIdentityTokenRejected
(Deny / authenticator exception). Anonymous + X509 tokens fall through to
SDK defaults.

LdapOpcUaUserAuthenticator (Host project) bridges to the same
ILdapAuthService that AddOtOpcUaAuth uses for Admin cookies / JWT, so a
single LDAP source-of-truth governs both Admin control plane and OPC UA
data plane. Program.cs registers LdapOptions + LdapAuthService +
IOpcUaUserAuthenticator on driver-role hosts; admin-only nodes are
unchanged.

OtOpcUaServerHostedService threads the resolved authenticator into
OpcUaApplicationHost so the seam respects Host DI.

10 new tests: 6 in OpcUaServer.Tests cover the pure HandleImpersonation
static method (success / denial / anonymous fallthrough / authenticator-
throw / null-username / Null authenticator); 4 in Host.IntegrationTests
cover the LdapOpcUaUserAuthenticator adapter (LDAP allow → Allow with
roles, LDAP deny → Deny, exception → backend-error denial, display-name
fallback). OpcUaServer suite is 40 / 40 green.

Closes #104. Unblocks Task 60 (dual-endpoint + ServiceLevel tests) once
#81 residual lands.

src/Server/ZB.MOM.WW.OtOpcUa.Host/OpcUa/OtOpcUaServerHostedService.cs

@@ -3,6 +3,7 @@ using Microsoft.Extensions.Hosting;
 using Microsoft.Extensions.Logging;
 using ZB.MOM.WW.OtOpcUa.Commons.OpcUa;
 using ZB.MOM.WW.OtOpcUa.OpcUaServer;
+using ZB.MOM.WW.OtOpcUa.OpcUaServer.Security;
 
 namespace ZB.MOM.WW.OtOpcUa.Host.OpcUa;
 
@@ -21,6 +22,7 @@ public sealed class OtOpcUaServerHostedService : IHostedService, IAsyncDisposabl
 {
     private readonly IConfiguration _configuration;
     private readonly DeferredAddressSpaceSink _deferredSink;
+    private readonly IOpcUaUserAuthenticator _userAuthenticator;
     private readonly ILoggerFactory _loggerFactory;
     private readonly ILogger<OtOpcUaServerHostedService> _logger;
 
@@ -30,10 +32,12 @@ public sealed class OtOpcUaServerHostedService : IHostedService, IAsyncDisposabl
     public OtOpcUaServerHostedService(
         IConfiguration configuration,
         DeferredAddressSpaceSink deferredSink,
+        IOpcUaUserAuthenticator userAuthenticator,
         ILoggerFactory loggerFactory)
     {
         _configuration = configuration;
         _deferredSink = deferredSink;
+        _userAuthenticator = userAuthenticator;
         _loggerFactory = loggerFactory;
         _logger = loggerFactory.CreateLogger<OtOpcUaServerHostedService>();
     }
@@ -44,7 +48,10 @@ public sealed class OtOpcUaServerHostedService : IHostedService, IAsyncDisposabl
         _configuration.GetSection("OpcUa").Bind(options);
 
         _server = new OtOpcUaSdkServer();
-        _appHost = new OpcUaApplicationHost(options, _loggerFactory.CreateLogger<OpcUaApplicationHost>());
+        _appHost = new OpcUaApplicationHost(
+            options,
+            _loggerFactory.CreateLogger<OpcUaApplicationHost>(),
+            _userAuthenticator);
 
         try
         {