ref(issue-ownership): allow all assignments when open membership is on

[cvxluo]

Issue ownership assignment permissions have a lot of edge cases around open membership / non-open membership orgs right now (more details). Proposed solution here is to allow all assignments when open membership is on, which significantly simplifies the number of cases and is more in line with user expectations. No changes for non-open membership (yet).

[github-actions]

Backend Test Failures

Failures on 3c0e53e in this run:

tests/sentry/integrations/slack/webhooks/actions/test_status.py::StatusActionTest::test_assign_issue_where_team_not_in_project — log

[gw1] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/integrations/slack/webhooks/actions/test_status.py:608: in test_assign_issue_where_team_not_in_project
    assert resp.data["text"].endswith("Cannot assign to a team without access to the project")
E   AssertionError: assert False
E    +  where False = <built-in method endswith of str object at 0x7f4c37873f30>('Cannot assign to a team without access to the project')
E    +    where <built-in method endswith of str object at 0x7f4c37873f30> = '[wise-liger] IntegrationError: Identity not found.'.endswith

tests/sentry/integrations/slack/webhooks/actions/test_status.py::StatusActionTest::test_assign_issue_where_team_not_in_project_through_unfurl — log

[gw1] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/integrations/slack/webhooks/actions/test_status.py:622: in test_assign_issue_where_team_not_in_project_through_unfurl
    assert resp.data["text"].endswith("Cannot assign to a team without access to the project")
E   AssertionError: assert False
E    +  where False = <built-in method endswith of str object at 0x7f95d871d170>('Cannot assign to a team without access to the project')
E    +    where <built-in method endswith of str object at 0x7f95d871d170> = '[smiling-newt] IntegrationError: Identity not found.'.endswith

tests/sentry/issues/endpoints/test_group_details.py::GroupUpdateTest::test_assign_unavailable_team — log

[gw0] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/issues/endpoints/test_group_details.py:619: in test_assign_unavailable_team
    assert response.status_code == 400, response.content
E   AssertionError: b'{"id":"35","shareId":null,"shortId":"BAR-1","title":"Hello world","culprit":null,"permalink":"http://testserver/orga...d":false,"count":"1","userCount":0,"firstSeen":"2026-03-30T18:36:08.097769Z","lastSeen":"2026-03-30T18:36:08.097763Z"}'
E   assert 200 == 400
E    +  where 200 = <Response status_code=200, "application/json">.status_code