Fix red develop bd642215dd7a

The merge lane found `develop` red at `bd642215dd7a38d730c832c0ba26da5c45440d94` on its own: no queued PR merges until it is fixed. Failing (tests and gate stages): - `tests/test_durable_enqueue_engine.py::test_the_same_workflow_id_enqueued_twice_through_the_seam_attaches_to_one_workflow` - `tests/test_tenant_gmail.py::test_the_uploaded_key_never_reaches_the_activity_log` - `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 `15525` on lane task 13523. 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.

Fixed the tenant_gmail activity-log flake (TestClient per-request loop cancelled queued capture writes; shim now drains them). PR #7921 queued; quick gate + 1365 area tests green. DBOS worker exits deferred to draft Task 13554. Memory: used mem-25288, mem-36807.