ref: fix types for test_event_attachment_details (#82091)

<!-- Describe your PR here. -->

pyproject.toml

@@ -321,7 +321,6 @@ module = [
     "tests.sentry.api.bases.test_team",
     "tests.sentry.api.endpoints.notifications.test_notification_actions_details",
     "tests.sentry.api.endpoints.notifications.test_notification_actions_index",
-    "tests.sentry.api.endpoints.test_event_attachment_details",
     "tests.sentry.api.helpers.test_group_index",
     "tests.sentry.api.test_authentication",
     "tests.sentry.api.test_base",

tests/sentry/api/endpoints/test_event_attachment_details.py

@@ -4,7 +4,7 @@
 from sentry.attachments.base import CachedAttachment
 from sentry.models.activity import Activity
 from sentry.models.eventattachment import EventAttachment
-from sentry.testutils.cases import APITestCase, PermissionTestCase
+from sentry.testutils.cases import APITestCase, PermissionTestCase, TestCase
 from sentry.testutils.helpers.datetime import before_now
 from sentry.testutils.helpers.features import with_feature
 from sentry.testutils.helpers.options import override_options
@@ -17,7 +17,7 @@
 ATTACHMENT_CONTENT = b"File contents here" * 10_000
 
 
-class CreateAttachmentMixin:
+class CreateAttachmentMixin(TestCase):
     def create_attachment(self, content: bytes | None = None, group_id: int | None = None):
         self.project = self.create_project()
         self.release = self.create_release(self.project, self.user)
@@ -151,6 +151,7 @@ def test_delete_activity_with_group(self):
         delete_activity = Activity.objects.get(type=ActivityType.DELETED_ATTACHMENT.value)
         assert delete_activity.project == self.project
         assert delete_activity.group_id == group_id
+        assert delete_activity.group is not None
         assert delete_activity.group.id == group_id