Fix red develop a977815e4971

The merge lane found `develop` red at `a977815e497171278d4c146971d33fc1f4ed290c` on its own: no queued PR merges until it is fixed. Failing (tests and gate stages): - `tests/billing/test_customer_billing.py::test_payment_link_recipients_name_exactly_who_the_send_reaches` - `tests/billing/test_customer_billing.py::test_payment_link_adds_the_billing_email_once_when_asked` - `api-unit-dc-test` The whole gate log: artifact `9106` on lane task 9462. 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.

Red develop came from two billing tests unpacking OutboundEmail as tuples. Task 9472 (#6771) already fixed it on develop; billing suites show 42 passed on 92c15c744. No PR needed.