feat(opcuaclient): opt-in server-certificate host-name check; NAT fixture keeps its PKI (#816, #818)

#816 finding: the SDK checks that the server certificate names the host only
when a session is created with checkDomain:true. The driver, the AdminUI
browser and Client.Shared all called the 8-argument
DefaultSessionFactory.CreateAsync, which forwards checkDomain:false — so no
host-name check ever ran (why a trusted NAT cert for *.invalid browsed fine
over an IP in #813). When enabled the SDK (Session.OpenAsync ->
ValidateDomains -> FindDomain) compares ConfiguredEndpoint.EndpointUrl.IdnHost
— the CONFIGURED host, because OpcUaEndpointSelection/EndpointSelector
rewrote the advertised one — against the cert's DNS/IP SANs; secured
sessions only, and the auto-accept handler accepts a mismatch.

Policy: certificateValidation.validateServerHostName, tri-state. Unset =
OpcUaCertificateValidationOptions.DefaultValidateServerHostName = false, the
pre-#816 behaviour, so nothing changes on upgrade; true = validate the
configured host; false = explicit NAT/port-publish opt-out. Threaded into the
driver, the AdminUI browser (same JSON) and the AdminUI form (a select that
keeps "unset" unset), Client.Shared ConnectionSettings.ValidateServerHostName
and the CLI --validate-host-name. A driver refusal names the opt-out.

#818: the NAT fixture's /app/pki is on the opc-plc-nat-pki volume (verified:
same thumbprint across --force-recreate), its PUBLIC cert is committed as
Docker/nat/opc-plc-nat-server.der, and NatPublishedEndpointBrowseTests run
every secure browse on a throwaway PKI (internal browser seam): refusal with
nothing trusted, positive with the cert trusted (unset/false host check), and
a BadCertificateHostNameInvalid refusal with the check on. The trusted cert is
OPCUA_NAT_SERVER_CERT or the committed one (skip on thumbprint mismatch);
OPCUA_NAT_SERVER_CERT_TRUSTED is retired. CI exports its throwaway NAT
container's public cert. ServerHostNameValidationTests pin the driver against
the plain opc-plc. MIN_EXECUTED not raised (needs a hosted run): +6 in
v2-integration (440 -> 446), documented in scripts/ci/README.md.

src/Client/ZB.MOM.WW.OtOpcUa.Client.Shared/Models/ConnectionSettings.cs

@@ -40,6 +40,15 @@ public sealed class ConnectionSettings
     /// </summary>
     public bool AutoAcceptCertificates { get; set; } = true;
 
+    /// <summary>
+    ///     Gitea #816 — require the server certificate to list the host of the URL being connected (the operator's
+    ///     host: discovery keeps it in place of the server-advertised one) among its subject-alternative names;
+    ///     a mismatch fails the connect with <c>BadCertificateHostNameInvalid</c>. Defaults to <c>false</c>, the
+    ///     behaviour before #816 (the SDK overload used never checked). Secured connections only. Leave it off behind
+    ///     NAT / a port publish / when connecting by an IP the certificate does not name.
+    /// </summary>
+    public bool ValidateServerHostName { get; set; }
+
     /// <summary>
     ///     Path to the certificate store. Defaults to <see cref="string.Empty"/>; the
     ///     consuming application configuration factory resolves the canonical path via