Fix red develop 98b0031c3073

The merge lane found `develop` red at `98b0031c3073e4cdfca58505f425227a733eb8b5` on its own: no queued PR merges until it is fixed. Failing (tests and gate stages): - `tests/token_billing/test_outbox.py::test_the_effect_runs_on_dbos_only_when_its_transaction_commits_and_a_failure_is_retried` - `api-unit-dc-test` The whole gate log: artifact `15022` on lane task 13170. Make `develop` pass its full gate again. Open a PR as usual: the merge lane merges a PR that fixes some of these and adds no failure even while `develop` is red.

Shared live-DBOS engine fixture now drains workflows (SHUTDOWN_DRAIN_SEC) before DBOS.destroy, closing the teardown race that crashed xdist worker gw4 in test_outbox. Quick gate green; PR #7836 queued in merge lane. Memory: none applied.