fix: Return users in project.all_members() (HumanSignal#5991)

#### Change has impacts in these area(s)
_(check all that apply)_
- [x] Backend (API)



### Describe the reason for change
project.all_members() returned member ids instead of user ids


#### What does this fix?
all_members() returns user ids now. 


#### What is the new behavior?
Data manager filters display the correct user list. 



#### What is the current behavior?
Data manager filters displayed empty user items. 

![image](https://github.com/HumanSignal/label-studio/assets/501780/67daf1d8-acf9-416e-9efc-9917872be121)

### What level of testing was included in the change?
_(check all that apply)_
- [ ] e2e
- [ ] integration
- [ ] unit

label_studio/projects/mixins.py

@@ -1,8 +1,11 @@
 from typing import Mapping, Optional
 
-from core.redis import start_job_async_or_sync
+from django.db.models import QuerySet
 from django.utils.functional import cached_property
 
+from core.redis import start_job_async_or_sync
+from users.models import User
+
 
 class ProjectMixin:
     def rearrange_overlap_cohort(self):
@@ -85,9 +88,9 @@ def _can_use_overlap(self):
         return True
 
     @cached_property
-    def all_members(self):
+    def all_members(self) -> QuerySet[User]:
         """
-        Returns all members of project
-        :return:
+        Returns all users of project
+        :return: List[User]
         """
-        return self.organization.members.values_list('user__id', flat=True)
+        return User.objects.filter(id__in=self.organization.members.values_list('user__id'))

label_studio/tests/test_project.py

@@ -1,6 +1,8 @@
 import json
-
 import pytest
+
+from django.db.models.query import QuerySet
+from users.models import User
 from tests.utils import make_project
 
 
@@ -29,3 +31,11 @@ def test_update_tasks_counters_and_task_states(business_client):
     ids = set(project.tasks.all().values_list('id', flat=True))
     obj = project._update_tasks_counters_and_task_states(ids, True, True, True)
     assert obj == 0
+
+@pytest.mark.django_db
+def test_project_all_members(business_client):
+    project = make_project({}, business_client.user, use_ml_backend=False)
+    members = project.all_members
+
+    assert isinstance(members, QuerySet)
+    assert isinstance(members.first(), User)