fix(adminui): register the /dev/context-menu-demo route only in DEBUG builds (R04 S7)

The dev-only demo page shipped routable in the production assembly. Its route
moves from @page to a code-behind [Route] under #if DEBUG, so a Release build
keeps the component type (nothing referencing it breaks) but has no /dev/*
route. The page-authorization guard classifies it under #if DEBUG, and
DevRouteGatingTests asserts each configuration's half (Debug: exactly the demo
route; Release: none) with a positive control. Both configurations pass.

Refs #745
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Pages/Dev/ContextMenuDemo.razor

@@ -1,4 +1,5 @@
-@page "/dev/context-menu-demo"
+@* The /dev/context-menu-demo route lives in ContextMenuDemo.razor.cs behind #if DEBUG (R04 S7), so a
+   Release build ships the component but no route to it. *@
 @* Dev-only host page to live-verify the reusable ContextMenu component. *@
 @attribute [Authorize(Policy = AdminUiPolicies.AuthenticatedRead)]
 @rendermode InteractiveServer

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Pages/Dev/ContextMenuDemo.razor.cs

@@ -0,0 +1,16 @@
+namespace ZB.MOM.WW.OtOpcUa.AdminUI.Components.Pages.Dev;
+
+/// <summary>
+///     Dev-only host page that live-verifies the reusable <c>ContextMenu</c> component. The route is
+///     compiled only into DEBUG builds (R04 S7): a Release (production) build keeps the component type —
+///     so nothing that references it breaks — but registers no <c>/dev/*</c> route, and the router never
+///     serves it.
+/// </summary>
+#if DEBUG
+[Microsoft.AspNetCore.Components.Route(RoutePath)]
+#endif
+public partial class ContextMenuDemo
+{
+    /// <summary>The DEBUG-only route.</summary>
+    public const string RoutePath = "/dev/context-menu-demo";
+}