Fix flaky test tests/token_billing/test_outbox.py::test_the_effect_runs_on_dbos_only_when_its_transaction_commits_and_a_failure_is_retried

The merge lane saw `tests/token_billing/test_outbox.py::test_the_effect_runs_on_dbos_only_when_its_transaction_commits_and_a_failure_is_retried` fail and then pass on a rerun of the same tree. - Test: `tests/token_billing/test_outbox.py::test_the_effect_runs_on_dbos_only_when_its_transaction_commits_and_a_failure_is_retried` - Tree: `e16d159d2f6d18dc8266fe130dcaef937c0a806f` - Lane task: 13353 - Flake record: 01a1a2a8-f97b-7121-b737-d30257431474 Make it deterministic; do not delete it unless it tests nothing.

Fixed the outbox live-engine flake: the shared DBOS test fixture now stops the LISTEN thread on its own thread before DBOS.destroy, closing a use-after-free in the connection teardown on free-threaded CPython. Quick gate green; PR #7874 queued to the lane. Filed draft 13385 for the other destroy sites. Memory: none applied.