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/StubInnovationLookup.cs

@@ -0,0 +1,34 @@
+using TarninaHub.Application.Abstractions;
+using TarninaHub.Application.Dtos;
+
+namespace TarninaHub.Tests.Stubs;
+
+/// <summary>Innovation lookup stub returning a fixed (or null) innovation.</summary>
+public sealed class StubInnovationLookup : IInnovationService
+{
+    private readonly InnovationResponse? _innovation;
+
+    public StubInnovationLookup(InnovationResponse? innovation)
+    {
+        _innovation = innovation;
+    }
+
+    public Task<IReadOnlyList<InnovationResponse>> ListAsync(
+        string? challengeId,
+        string? stage,
+        string? query,
+        bool onlyPublished,
+        CancellationToken cancellationToken) => Task.FromResult<IReadOnlyList<InnovationResponse>>([]);
+
+    public Task<InnovationResponse?> GetAsync(string innovationId, CancellationToken cancellationToken) =>
+        Task.FromResult(_innovation);
+
+    public Task<InnovationResponse> CreateAsync(
+        InnovationRequest request,
+        CancellationToken cancellationToken) => throw new NotSupportedException();
+
+    public Task<InnovationResponse?> UpdateAsync(
+        string innovationId,
+        InnovationRequest request,
+        CancellationToken cancellationToken) => throw new NotSupportedException();
+}