bug: fix bug and update test (#4341)

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# Description

Please include a summary of the changes and the related issue. Please
also include relevant motivation and context. List any dependencies that
are required for this change.

Closes #4340 

**Type of change**

(Please delete options that are not relevant. Remember to title the PR
according to the type of change)

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)

**How Has This Been Tested**

(Please describe the tests that you ran to verify your changes. And
ideally, reference `tests`)

- [ ] Test A
- [ ] Test B

**Checklist**

- [ ] I followed the style guidelines of this project
- [ ] I did a self-review of my code
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I filled out [the contributor form](https://tally.so/r/n9XrxK)
(see text above)
- [ ] I have added relevant notes to the `CHANGELOG.md` file (See
https://keepachangelog.com/)

src/argilla/client/feedback/utils/assignment.py

@@ -241,7 +241,7 @@ def assign_workspaces(
                     except:
                         pass
 
-                wk_assignments[workspace_name] = [User.from_id(user).username for user in workspace.users]
+                wk_assignments[workspace_name] = [User.from_id(user.id).username for user in workspace.users]
 
             continue
 
@@ -257,6 +257,6 @@ def assign_workspaces(
             except:
                 pass
 
-        wk_assignments[workspace_name] = [User.from_id(user).username for user in workspace.users]
+        wk_assignments[workspace_name] = [User.from_id(user.id).username for user in workspace.users]
 
     return wk_assignments

tests/unit/client/feedback/utils/test_assignment.py

@@ -57,9 +57,16 @@ def _factory(*args, **kwargs):
         mock = Mock(spec=Workspace)
         mock.users = []
 
+        def create_mock_user(user_id):
+            user_mock = Mock()
+            user_mock.id = user_id
+            return user_mock
+
         def add_user(user_id):
-            if user_id not in mock.users:
-                mock.users.append(user_id)
+            # Check if a user with this ID already exists in the list
+            if not any(user.id == user_id for user in mock.users):
+                mock_user = create_mock_user(user_id)
+                mock.users.append(mock_user)
 
         mock.add_user.side_effect = add_user