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/StubIdeaAssistant.cs

@@ -0,0 +1,14 @@
+using TarninaHub.Application.Abstractions;
+using TarninaHub.Application.Dtos;
+
+namespace TarninaHub.Tests.Stubs;
+
+/// <summary>No-op IIdeaAssistant stub for controller tests.</summary>
+public sealed class StubIdeaAssistant : IIdeaAssistant
+{
+    public Task<IdeaAssistResponse?> AssistAsync(
+        string ideaId,
+        IdeaAssistRequest request,
+        CancellationToken cancellationToken) =>
+        Task.FromResult<IdeaAssistResponse?>(new IdeaAssistResponse([], "opis", false));
+}