Fix: enforce set_permissions shape via a nested DRF serializer (#14119)

SetPermissionsSerializer was a bare DictField, and the legacy bulk_edit
set_permissions parameter bypassed even that by hand-calling
validate_set_permissions() on an unchecked dict. A bool or a non-list in
place of users/groups crashed with a raw TypeError instead of a 400.

It is now a nested serializer (view/change, each with users/groups as
lists of integers), used for owned-object create/update, the legacy
bulk_edit set_permissions parameter, and bulk_edit_objects permissions.
Unknown action keys are rejected: previously a typo like "veiw" was
silently dropped, leaving an empty permission set that could clear
existing grants. An explicit set_permissions null (an owner-only change)
remains a no-op, and an empty bulk_edit_objects permissions dict is still
rejected.

src/documents/tests/test_api_permissions.py

@@ -1499,6 +1499,110 @@ class TestBulkEditObjectPermissions(APITestCase):
         self.assertEqual(response.status_code, status.HTTP_403_FORBIDDEN)
         self.assertTrue(Tag.objects.filter(pk=self.t1.id).exists())
 
+    def test_bulk_object_set_permissions_rejects_empty_permissions(self) -> None:
+        """
+        GIVEN:
+            - Existing objects
+        WHEN:
+            - bulk_edit_objects API endpoint is called with set_permissions
+              operation and an empty permissions dict
+        THEN:
+            - Validation fails rather than silently applying a no-op
+        """
+        response = self.client.post(
+            "/api/bulk_edit_objects/",
+            json.dumps(
+                {
+                    "objects": [self.t1.id],
+                    "object_type": "tags",
+                    "operation": "set_permissions",
+                    "permissions": {},
+                },
+            ),
+            content_type="application/json",
+        )
+
+        self.assertEqual(response.status_code, status.HTTP_400_BAD_REQUEST)
+
+    def test_bulk_object_set_permissions_rejects_non_dict_permissions(self) -> None:
+        """
+        GIVEN:
+            - Existing objects
+        WHEN:
+            - bulk_edit_objects API endpoint is called with set_permissions
+              operation and a non-dict permissions value
+        THEN:
+            - Validation fails rather than crashing
+        """
+        response = self.client.post(
+            "/api/bulk_edit_objects/",
+            json.dumps(
+                {
+                    "objects": [self.t1.id],
+                    "object_type": "tags",
+                    "operation": "set_permissions",
+                    "permissions": False,
+                },
+            ),
+            content_type="application/json",
+        )
+
+        self.assertEqual(response.status_code, status.HTTP_400_BAD_REQUEST)
+
+    def test_bulk_object_set_permissions_rejects_unknown_action(self) -> None:
+        """
+        GIVEN:
+            - Existing objects
+        WHEN:
+            - bulk_edit_objects API endpoint is called with set_permissions
+              operation and an unrecognized permission action name
+        THEN:
+            - Validation fails rather than silently no-oping
+        """
+        response = self.client.post(
+            "/api/bulk_edit_objects/",
+            json.dumps(
+                {
+                    "objects": [self.t1.id],
+                    "object_type": "tags",
+                    "operation": "set_permissions",
+                    "permissions": {"not_a_real_action": {"users": [self.user1.id]}},
+                },
+            ),
+            content_type="application/json",
+        )
+
+        self.assertEqual(response.status_code, status.HTTP_400_BAD_REQUEST)
+
+    def test_bulk_object_set_permissions_null_users_clears_users(self) -> None:
+        """
+        GIVEN:
+            - An object a user has view permission on
+        WHEN:
+            - bulk_edit_objects API endpoint is called with set_permissions
+              operation, merge off, and an explicit null for the view users
+        THEN:
+            - Request succeeds and null is treated as an empty user list,
+              so the existing view permission is removed
+        """
+        assign_perm("view_tag", self.user1, self.t1)
+
+        response = self.client.post(
+            "/api/bulk_edit_objects/",
+            json.dumps(
+                {
+                    "objects": [self.t1.id],
+                    "object_type": "tags",
+                    "operation": "set_permissions",
+                    "permissions": {"view": {"users": None}},
+                },
+            ),
+            content_type="application/json",
+        )
+
+        self.assertEqual(response.status_code, status.HTTP_200_OK)
+        self.assertNotIn(self.user1, get_users_with_perms(self.t1))
+
     def test_bulk_edit_object_permissions_validation(self) -> None:
         """
         GIVEN: