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/Controllers/IdeasControllerTests.cs

@@ -0,0 +1,54 @@
+using System.Security.Claims;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+using TarninaHub.Application.Abstractions;
+using TarninaHub.Application.Dtos;
+using TarninaHub.Controllers;
+using TarninaHub.Tests.Stubs;
+
+namespace TarninaHub.Tests.Controllers;
+
+public class IdeasControllerTests
+{
+    private static IdeasController CreateController(IIdeaService ideaService, INotificationService notifications)
+    {
+        var controller = new IdeasController(
+            ideaService,
+            new StubIdeaAssistant(),
+            notifications);
+        var identity = new ClaimsIdentity(
+            [new Claim("sub", "user:tester"), new Claim("role", "user")],
+            authenticationType: "test");
+
+        controller.ControllerContext = new ControllerContext
+        {
+            HttpContext = new DefaultHttpContext { User = new ClaimsPrincipal(identity) },
+        };
+
+        return controller;
+    }
+
+    [Fact]
+    public async Task Create_NotifiesStaffAndReturnsCreated()
+    {
+        var ideaService = new StubIdeaService { Idea = SampleIdea() };
+        var notifications = new StubNotificationService();
+        var controller = CreateController(ideaService, notifications);
+
+        var result = await controller.Create(
+            new CreateIdeaRequest("Nowy pomysł", "Opis nowego pomysłu mieszkańca.", null),
+            CancellationToken.None);
+
+        Assert.IsType<CreatedAtActionResult>(result.Result);
+        Assert.Equal(1, notifications.StaffNotifications);
+    }
+
+    private static IdeaResponse SampleIdea() => new(
+        "idea:new",
+        "Nowy pomysł",
+        "Opis nowego pomysłu mieszkańca.",
+        "user:tester",
+        "draft",
+        null,
+        DateTimeOffset.UtcNow);
+}