Flaky test: tests/sentry/api/helpers/test_group_index.py::MergeGroupsTest::test_multiple_groups_same_project

Failing test: `tests/sentry/api/helpers/test_group_index.py::MergeGroupsTest::test_multiple_groups_same_project`
Sentry sha: ab471d2d1ede60c8a8c1e3911b78a71a83261eae
Run: https://github.com/getsentry/sentry/actions/runs/22271637063

```
tests/sentry/api/helpers/test_group_index.py:515: in test_multiple_groups_same_project
assert [group.id for group in call_args[0]] == group_ids
E assert [424, 423] == [423, 424]
E 
E At index 0 diff: 424 != 423
E 
E Full diff:
E [
E + 424,
E 423,
E - 424,
E ]
```

Start with tests/sentry/api/helpers/test_group_index.py::MergeGroupsTest::test_multiple_groups_same_project and reproduce it using the linked Actions run or the named test. Inspect why the group IDs arrive as [424, 423] instead of [423, 424]; done means the test no longer fails intermittently because of this ordering difference.

Written by the indexing model from the issue text.