test(backend): cover step 17 modules and extract stubs

Add tests for partnership workflow, middleman adaptation and idea notifications; extract reusable test stubs into separate files; add architecture guard forbidding object-valued dictionaries in data contracts.

tests/TarninaHub.Tests/Stubs/StubNotificationService.cs

@@ -0,0 +1,40 @@
+using TarninaHub.Application.Abstractions;
+using TarninaHub.Application.Dtos;
+
+namespace TarninaHub.Tests.Stubs;
+
+/// <summary>No-op INotificationService stub for controller tests.</summary>
+public sealed class StubNotificationService : INotificationService
+{
+    public int StaffNotifications { get; private set; }
+
+    public int UserNotifications { get; private set; }
+
+    public Task NotifyStaffAsync(
+        string type,
+        IReadOnlyDictionary<string, string?> payload,
+        CancellationToken cancellationToken)
+    {
+        StaffNotifications++;
+        return Task.CompletedTask;
+    }
+
+    public Task NotifyUserAsync(
+        string userId,
+        string type,
+        IReadOnlyDictionary<string, string?> payload,
+        CancellationToken cancellationToken)
+    {
+        UserNotifications++;
+        return Task.CompletedTask;
+    }
+
+    public Task<IReadOnlyList<NotificationResponse>> ListForUserAsync(
+        string userId,
+        CancellationToken cancellationToken) => Task.FromResult<IReadOnlyList<NotificationResponse>>([]);
+
+    public Task<NotificationResponse?> MarkReadAsync(
+        string notificationId,
+        string userId,
+        CancellationToken cancellationToken) => Task.FromResult<NotificationResponse?>(null);
+}