refactor(event-subscriber): drop the hoofdObject fallback (refs #153)

For a `resource: object` notification Objecten sends the object as both hoofdObject and
resourceUrl — the object *is* the main resource — so `HoofdObject ?? ResourceUrl` was a
branch that can never take its left side and that no test could distinguish. It came across
from the zaken path, where hoofdObject genuinely differed (the zaak behind a status).

Tests unchanged and green.

services/event-subscriber/EventSubscriber.Application/Notification.cs

@@ -16,15 +16,15 @@ public sealed record Notification(
     string Kanaal,
     string Resource,
     string Actie,
-    Uri ResourceUrl,
-    Uri? HoofdObject = null)
+    Uri ResourceUrl)
 {
     /// <summary>A register record written to Objecten — <c>create</c> on submit, <c>update</c> on
     /// approval, since the ACL upserts the same object for a registration (§8.6).</summary>
     public bool IsRegisterRecordWritten =>
         Kanaal == "objecten" && Resource == "object" && Actie is "create" or "update";
 
-    /// <summary>The object holding the register record. Objecten sets both fields to the object;
-    /// <c>hoofdObject</c> is the main resource by definition, so prefer it.</summary>
-    public Uri ObjectUrl => HoofdObject ?? ResourceUrl;
+    /// <summary>The object holding the register record. For a <c>resource: object</c> notification
+    /// Objecten sends the object as both <c>hoofdObject</c> and <c>resourceUrl</c> — the object is
+    /// the main resource — so the notification's own <c>hoofdObject</c> is not modelled.</summary>
+    public Uri ObjectUrl => ResourceUrl;
 }