Refactor user ID access in SignalR hubs and add online user tracking

Introduces IHubUserAccessor and its implementation to centralize user ID retrieval from SignalR HubCallerContext, replacing duplicated logic in ChatsHub, FriendsHub, and PresenceHub. Moves extension and mapping files to a Common directory, adds UserToUserDtoMappingAction for online status mapping, and implements OnlineUserStore with tests for tracking online users. Updates dependency injection and test code to use the new abstractions.

Govor.Application.Tests/Services/UserOnlineStatus/OnlineUserStoreTests.cs

@@ -0,0 +1,82 @@
+using Govor.Application.Interfaces.UserOnlineStatus;
+using Govor.Application.Services.UserOnlineStatus;
+
+namespace Govor.Application.Tests.Services.UserOnlineStatus;
+
+[TestFixture]
+public class OnlineUserStoreTests
+{
+    private IOnlineUserStore _store;
+    private Guid _userId1;
+    private Guid _userId2;
+
+    [SetUp]
+    public void Setup()
+    {
+        _store = new OnlineUserStore();
+        _userId1 = Guid.NewGuid();
+        _userId2 = Guid.NewGuid();
+    }
+
+    [Test]
+    public void SetOnlineUser_UserIsMarkedOnline()
+    {
+        // Act 
+        _store.SetOnlineUser(_userId1);
+
+        // Assert 
+        Assert.That(_store.IsOnline(_userId1), Is.True);
+    }
+
+    [Test]
+    public void SetOfflineUser_UserIsNoLongerOnline()
+    {
+        // Act 
+        _store.SetOnlineUser(_userId1);
+        _store.SetOfflineUser(_userId1);
+        
+        // Assert 
+        Assert.That(_store.IsOnline(_userId1), Is.False);
+    }
+
+    [Test]
+    public void IsOnline_ReturnsFalse_ForUnknownUser()
+    {
+        // Act & Assert 
+        Assert.That(_store.IsOnline(Guid.NewGuid()), Is.False);
+    }
+
+    [Test]
+    public void GetAllOnlineUsers_ReturnsAllCurrentlyOnlineUsers()
+    {
+        // Arrange  
+        _store.SetOnlineUser(_userId1);
+        _store.SetOnlineUser(_userId2);
+
+        // Act 
+        var onlineUsers = _store.GetAllOnlineUsers();
+    
+        // Assert 
+        Assert.That(onlineUsers, Is.EquivalentTo(new[] { _userId1, _userId2 }));
+    }
+
+    [Test]
+    public void SetOnlineUser_Twice_DoesNotThrow()
+    {
+        // Act & Assert  
+        Assert.DoesNotThrow(() =>
+        {
+            _store.SetOnlineUser(_userId1);
+            _store.SetOnlineUser(_userId1); 
+        });
+
+        Assert.That(_store.IsOnline(_userId1), Is.True);
+    }
+
+    [Test]
+    public void SetOfflineUser_ForUnknownUser_DoesNotThrow()
+    {
+        // Act & Assert 
+        Assert.DoesNotThrow(() => _store.SetOfflineUser(Guid.NewGuid()));
+    }
+}