feat(acl): anonymous becomes a normal grantable principal (#520 D3)

Anonymous sessions used to short-circuit in HandleImpersonation — "let the SDK's
default validation stand" — so they always connected and always carried no roles.
Now admission and authority are separated:

  admission  -> OpcUa:AllowAnonymous, the HOST's decision (it owns token policies)
  authority  -> the authenticator, resolving the reserved principal "anonymous"
                through the SAME grant path a key's principal goes through

So anonymous can be granted WRITE, not merely tolerated as read-only, and it
arrives at the downstream gates as the same RoleCarryingUserIdentity a key
produces — which is the real payoff: no gate carries an anonymous-shaped branch,
so there is no bypass path to get wrong.

AllowAnonymous defaults to TRUE on purpose. Defaulting it false would silently
deny every client without a key the moment this deploys, including ScadaBridge's
Data Connection Layer in another repo. Making keys mandatory is a rollout
decision, not an upgrade side effect. When false, the Anonymous token policy is
withheld so a client is refused at DISCOVERY rather than only at activation, and
activation is rejected independently in case a client attempts a token type the
endpoint never advertised.

Two fail-closed choices, both falsified:
- A failure to read anonymous's grants DENIES rather than admitting with no
  roles. Admitting is the pre-#520 behaviour, and defaulting to it exactly when
  the server cannot determine what anonymous may do stops being harmless once
  reads are gated. Making it admit reds Anonymous_denies_when_grants_cannot_be_read.
- A key may not authenticate as the reserved "anonymous" principal, or a
  credentialled client silently inherits whatever anonymous was granted.

AuthenticateAnonymousAsync is a DEFAULT interface implementation returning
admit-with-no-roles, i.e. exactly the pre-#520 behaviour, so
NullOpcUaUserAuthenticator and every test double keep working rather than being
forced to reimplement it — important because driver-only nodes run the Null one
and would otherwise black out. Note a default interface method is only reachable
through the interface-typed reference, which is how OpcUaApplicationHost holds it.

An existing test asserted the old fall-through and correctly went red; it is
rewritten to pin the new contract rather than deleted, with the reason recorded.

OpcUaServer 366, Host 42 green.

Claude-Session: https://claude.ai/code/session_015p7wGqy3YpZNCpDzTpGMKo

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/OpcUaApplicationHostSecurityTests.cs

@@ -88,6 +88,20 @@ public sealed class OpcUaApplicationHostSecurityTests : IDisposable
         userName.SecurityPolicyUri.ShouldBe(SecurityPolicies.Basic256Sha256);
     }
 
+    /// <summary>
+    ///     With <c>OpcUa:AllowAnonymous</c> false the Anonymous policy is not advertised, so a client is
+    ///     refused at discovery rather than only at activation — but UserName survives, which is what
+    ///     makes the server key-only rather than unreachable (#520).
+    /// </summary>
+    [Fact]
+    public void BuildUserTokenPolicies_withholds_anonymous_when_disallowed()
+    {
+        var tokens = OpcUaApplicationHost.BuildUserTokenPolicies(allowAnonymous: false).ToList();
+
+        tokens.ShouldNotContain(t => t.TokenType == UserTokenType.Anonymous);
+        tokens.ShouldContain(t => t.TokenType == UserTokenType.UserName);
+    }
+
     /// <summary>
     /// Verifies that StartAsync populates ServerConfiguration with all enabled security profiles.
     /// </summary>