Refactor group existence checks and add group repo tests

Renamed IGroupsExist and related methods from Exists to Exist for consistency. Added integration tests for GroupRepository. Introduced entity configurations for ChatGroup, GroupAdmins, GroupInvitation, and updated GroupMembership configuration. Updated usages and tests to match new method names and improved equality checks for ChatGroup.

Govor.Core/Models/ChatGroup.cs

@@ -11,4 +11,16 @@ public class ChatGroup
     public List<GroupAdmins> Admins { get; set; } = new();
     public List<GroupMembership> Members { get; set; } = new();
     public List<GroupInvitation> InviteCodes { get; set; } = new();
+
+    public override bool Equals(object? obj)
+    {
+        ChatGroup chatGroup = obj as ChatGroup;
+        
+        return Id == chatGroup.Id &&
+               Name == chatGroup.Name &&
+               Description == chatGroup.Description &&
+               ImageId == chatGroup.ImageId &&
+               IsChannel == chatGroup.IsChannel &&
+               IsPrivate == chatGroup.IsPrivate;
+    }
 }