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/StubApplicationService.cs

@@ -0,0 +1,27 @@
+using TarninaHub.Application.Abstractions;
+using TarninaHub.Application.Dtos;
+
+namespace TarninaHub.Tests.Stubs;
+
+/// <summary>Configurable IApplicationService stub for controller tests.</summary>
+public sealed class StubApplicationService : IApplicationService
+{
+    private readonly CreateApplicationResult _createResult;
+
+    public StubApplicationService(CreateApplicationResult createResult)
+    {
+        _createResult = createResult;
+    }
+
+    public Task<CreateApplicationResult> CreateAsync(
+        string userId,
+        CreateApplicationRequest request,
+        CancellationToken cancellationToken) => Task.FromResult(_createResult);
+
+    public Task<ApplicationResponse?> GetAsync(string applicationId, CancellationToken cancellationToken) =>
+        Task.FromResult(_createResult.Application);
+
+    public Task<IReadOnlyList<ApplicationResponse>> ListForAuthorAsync(
+        string authorId,
+        CancellationToken cancellationToken) => Task.FromResult<IReadOnlyList<ApplicationResponse>>([]);
+}