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

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

@@ -43,15 +43,25 @@ public sealed class ConfigDbApiKeyStore : IApiKeyStore
 
         if (key is null) return null;
 
+        return new ApiKeyLookup(key, await LoadClusterGrantsAsync(db, key.Principal, ct).ConfigureAwait(false));
+    }
+
+    /// <inheritdoc />
+    public async Task<IReadOnlyList<NodeAcl>> FindGrantsAsync(string principal, CancellationToken ct)
+    {
+        await using var db = await _factory.CreateDbContextAsync(ct).ConfigureAwait(false);
+        return await LoadClusterGrantsAsync(db, principal, ct).ConfigureAwait(false);
+    }
+
+    private static async Task<IReadOnlyList<NodeAcl>> LoadClusterGrantsAsync(
+        OtOpcUaConfigDbContext db, string principal, CancellationToken ct)
+    {
         // Only cluster-scoped rows can confer an interim role (see ApiKeyInterimRoleMapper), so only
         // those are fetched. Stage 5 widens this to the full grant set for per-node evaluation.
-        var grants = await db.NodeAcls
+        return await db.NodeAcls
             .AsNoTracking()
-            .Where(a => a.Principal == key.Principal
-                        && a.ScopeKind == NodeAclScopeKind.Cluster)
+            .Where(a => a.Principal == principal && a.ScopeKind == NodeAclScopeKind.Cluster)
             .ToListAsync(ct)
             .ConfigureAwait(false);
-
-        return new ApiKeyLookup(key, grants);
     }
 }