Resolve the group services lazily in the authentication provider

Jellyfin's UserManager constructor-injects every IAuthenticationProvider, so
building IUserManager forced SharedAccountAuthenticationProvider to be built
first. That provider eagerly required IGroupService and IDynamicGroupService,
both of which need IUserManager, and the container refused to start the server
with "a circular dependency was detected".

Take the two group services as Lazy<T> and dereference them at authentication
time instead. Nobody can log in before the host is up, so the deferred lookup
is always safe. Microsoft's container has no built-in Lazy<T> support, hence
the explicit factory registrations.

The accompanying test builds the service graph through a stand-in that mimics
UserManager's constructor shape and validates it on build, so a reintroduced
cycle fails in CI rather than at server startup.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Jellyfin.Plugin.WatchedTogether/ServiceRegistrator.cs

@@ -1,3 +1,4 @@
+using System;
 using Jellyfin.Plugin.WatchedTogether.Auth;
 using Jellyfin.Plugin.WatchedTogether.Services;
 using MediaBrowser.Controller;
@@ -20,6 +21,14 @@ public class ServiceRegistrator : IPluginServiceRegistrator
         serviceCollection.AddSingleton<IProvisioningService, ProvisioningService>();
         serviceCollection.AddSingleton<IDynamicGroupService, DynamicGroupService>();
 
+        // The auth provider takes these lazily so the container can build it while IUserManager is
+        // still under construction; see SharedAccountAuthenticationProvider's constructor remarks.
+        // Microsoft's container has no built-in Lazy<T> support, so the factories are explicit.
+        serviceCollection.AddSingleton(
+            provider => new Lazy<IGroupService>(provider.GetRequiredService<IGroupService>));
+        serviceCollection.AddSingleton(
+            provider => new Lazy<IDynamicGroupService>(provider.GetRequiredService<IDynamicGroupService>));
+
         // Discovered by Jellyfin and matched to shared accounts via User.AuthenticationProviderId.
         serviceCollection.AddSingleton<IAuthenticationProvider, SharedAccountAuthenticationProvider>();