Hosting AddNodeManager(namespaceUri, build) always creates a root folder named "ReferenceServer"

[romanett]

Summary

The hosting AddNodeManager(namespaceUri, Action<INodeManagerBuilder> build) overload always creates a FolderState named ReferenceServer under the Objects folder before the build callback runs. The name is a hard-coded constant in the internal FluentNodeManager, it is not documented, it cannot be configured, and every consumer of the hosting fluent API ends up with an extra top-level object called ReferenceServer in their address space regardless of what their server is.

Where

src/Opc.Ua.Server/Hosting/FluentNodeManagerFactory.cs:

public override async ValueTask CreateAddressSpaceAsync(...)
{
    ...
    FolderState root = CreateRootFolder(namespaceIndex);          // line 103
    await AddPredefinedNodeAsync(root, cancellationToken).ConfigureAwait(false);
    NodeManagerBuilder builder = CreateFluentBuilder(namespaceIndex);
    m_build(builder);
    ...
}

private static FolderState CreateRootFolder(ushort namespaceIndex)
{
    const string browseName = "ReferenceServer";                  // line 118
    var root = new FolderState(null)
    {
        NodeId = new NodeId(browseName, namespaceIndex),
        BrowseName = new QualifiedName(browseName, namespaceIndex),
        DisplayName = new LocalizedText(browseName),
        TypeDefinitionId = ObjectTypeIds.FolderType
    };
    root.AddReference(ReferenceTypeIds.Organizes, true, ObjectIds.ObjectsFolder);
    return root;
}

Introduced in #3957. The only in-tree consumer that wants this exact folder is AddReferenceServer (OpcUaServerBuilderExtensions.cs ~line 1268), which registers AddNodeManager("http://opcfoundation.org/UA/ReferenceServer", nm => nm.Node("ReferenceServer")), i.e. the demo preset leaked its root-folder name into the general-purpose API. docs/NodeManagers.md describes the hosting AddNodeManager overload without mentioning that a root folder is created at all.

Why it matters

• A server built with services.AddOpcUa().AddServer().AddNodeManager("urn:mycompany:plant", b => ...) exposes an object called ReferenceServer under Objects. There is no way to rename, retype or suppress it without dropping to a hand-written FluentNodeManagerBase subclass, which defeats the point of the one-shot overload.
• FluentNodeManager and FluentNodeManagerFactory are internal sealed, so the folder cannot be replaced by subclassing either.
• The NodeId of the folder is the string "ReferenceServer" in the caller's namespace, so two managers registered through this overload in different namespaces both carry a node with that identifier.
• Since Fluent: publish Configure-created nodes' references on foreign-manager nodes (Objects folder placement) #4331 the builder can place nodes under Objects itself (UnderObjectsFolder() / OrganizedBy(...) + CompleteConfigureAsync), so the implicit root folder is no longer needed as the anchor for build-created nodes.

Proposal

1. Stop creating an implicit root folder by default. The build callback already has everything it needs to create and place its own root (builder.CreateInstance<FolderState>(...).Configure(n => n.UnderObjectsFolder()), or AddObject on an existing node).
2. If a convenience root is still wanted, make it opt-in and named by the caller, for example an overload AddNodeManager(namespaceUri, rootBrowseName, build) or an options object (RootFolder = new QualifiedName("Plant", ns)), with the folder handed to the callback as the builder's root node rather than looked up by a magic browse name.
3. Move the ReferenceServer folder creation into AddReferenceServer, which is the only place that name belongs.
4. Document the behaviour of the overload in docs/NodeManagers.md either way.

Affected tests: tests/Opc.Ua.Server.Tests/Hosting/ServerFluentApiHostingTests.cs (line ~587 does builder.Node("ReferenceServer") against the implicit folder) and OpcUaServerBuilderExtensionsCoverageTests.cs (AddReferenceServer*).

[marcschier]

Fix in #4389?