fix: correct notification template filter links

Use explicit Razor interpolation for create and edit URLs and cover the rendered link syntax with a regression test.

Ref: IT-1039

HrynCo.NotificationService.Web/Views/AdminTemplates/Index.cshtml

@@ -14,7 +14,7 @@
 
 <div class="page-header">
     <h2><i class="bi bi-envelope-paper"></i> Email Templates</h2>
-    <a href="/admin/templates/create@filterQuery" class="btn btn-primary btn-sm">
+    <a href="/admin/templates/create@(filterQuery)" class="btn btn-primary btn-sm">
         <i class="bi bi-plus-lg me-1"></i> Create New Template
     </a>
 </div>
@@ -146,7 +146,7 @@ else
                             <td>@t.LanguageCode</td>
                             <td>@t.Subject</td>
                             <td class="text-end">
-                                <a href="/admin/templates/@t.ServiceName/@t.Key/@t.LanguageCode@filterQuery"
+                                <a href="/admin/templates/@t.ServiceName/@t.Key/@t.LanguageCode@(filterQuery)"
                                    class="btn btn-sm btn-outline-primary me-1">
                                     <i class="bi bi-pencil"></i> Edit
                                 </a>