Fix review findings: assert grant-access preconditions, remove unused import

- Add StatusCode assertions on all GrantAccessDto calls so test setup
  failures surface clearly instead of causing confusing Forbidden errors
- Remove unused System.Collections.Concurrent import from SignalRTestHelper

Author: Chris0Jeky

backend/tests/Taskdeck.Api.Tests/BoardsHubIntegrationTests.cs

@@ -168,9 +168,11 @@ public async Task LeaveBoard_ShouldRemoveUserFromPresence()
 
         using var client2 = _factory.CreateClient();
         var user2 = await ApiTestHarness.AuthenticateAsync(client2, "hub-leave2");
-        await client1.PostAsJsonAsync(
+        var grantResponse = await client1.PostAsJsonAsync(
             $"/api/boards/{board.Id}/access",
             new GrantAccessDto(board.Id, user2.UserId, UserRole.Editor));
+        grantResponse.StatusCode.Should().Be(HttpStatusCode.OK,
+            "granting board access is a precondition for this test");
 
         // Observer (user1) joins board
         var observerEvents = new EventCollector<BoardPresenceSnapshot>();
@@ -209,9 +211,11 @@ public async Task AbruptDisconnect_ShouldCleanUpPresence()
 
         using var client2 = _factory.CreateClient();
         var user2 = await ApiTestHarness.AuthenticateAsync(client2, "hub-disc2");
-        await client1.PostAsJsonAsync(
+        var grantResponse = await client1.PostAsJsonAsync(
             $"/api/boards/{board.Id}/access",
             new GrantAccessDto(board.Id, user2.UserId, UserRole.Editor));
+        grantResponse.StatusCode.Should().Be(HttpStatusCode.OK,
+            "granting board access is a precondition for this test");
 
         // User1 connects and joins
         var events1 = new EventCollector<BoardPresenceSnapshot>();
@@ -252,9 +256,11 @@ public async Task MultipleUsersOnSameBoard_ShouldSeeAllMembersInPresence()
 
         using var client2 = _factory.CreateClient();
         var user2 = await ApiTestHarness.AuthenticateAsync(client2, "hub-multi2");
-        await client1.PostAsJsonAsync(
+        var grantResponse = await client1.PostAsJsonAsync(
             $"/api/boards/{board.Id}/access",
             new GrantAccessDto(board.Id, user2.UserId, UserRole.Editor));
+        grantResponse.StatusCode.Should().Be(HttpStatusCode.OK,
+            "granting board access is a precondition for this test");
 
         var events1 = new EventCollector<BoardPresenceSnapshot>();
         await using var conn1 = SignalRTestHelper.CreateBoardsHubConnection(_factory, user1.Token);

backend/tests/Taskdeck.Api.Tests/Support/SignalRTestHelper.cs

@@ -1,4 +1,3 @@
-using System.Collections.Concurrent;
 using Microsoft.AspNetCore.SignalR.Client;
 using Microsoft.AspNetCore.Mvc.Testing;