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/StubSubmissionService.cs

@@ -0,0 +1,41 @@
+using TarninaHub.Application.Abstractions;
+using TarninaHub.Application.Dtos;
+
+namespace TarninaHub.Tests.Stubs;
+
+/// <summary>Configurable ISubmissionService stub for controller tests.</summary>
+public sealed class StubSubmissionService : ISubmissionService
+{
+    public SubmissionResponse? Current { get; set; }
+
+    public SubmissionResponse? UpdateResult { get; set; }
+
+    public string? LastRequestedStatus { get; private set; }
+
+    public Task<SubmissionResponse> CreateAsync(
+        string authorId,
+        CreateSubmissionRequest request,
+        CancellationToken cancellationToken) => Task.FromResult(Current ?? throw new InvalidOperationException());
+
+    public Task<PagedResponse<SubmissionResponse>> ListAsync(
+        string viewerId,
+        bool isStaff,
+        string? status,
+        int page,
+        int pageSize,
+        CancellationToken cancellationToken) => Task.FromResult(
+            new PagedResponse<SubmissionResponse>([], page, pageSize, 0));
+
+    public Task<SubmissionResponse?> GetAsync(string submissionId, CancellationToken cancellationToken) =>
+        Task.FromResult(Current);
+
+    public Task<SubmissionResponse?> UpdateStatusAsync(
+        string submissionId,
+        string newStatus,
+        CancellationToken cancellationToken)
+    {
+        LastRequestedStatus = newStatus;
+
+        return Task.FromResult(UpdateResult);
+    }
+}