Fix flaky test apps/api/tests/test_tenancy_seam_reads.py::test_the_criteria_reaches_an_entity_that_is_only_joined

The merge lane saw `apps/api/tests/test_tenancy_seam_reads.py::test_the_criteria_reaches_an_entity_that_is_only_joined` fail and then pass on a rerun of the same tree. - Test: `apps/api/tests/test_tenancy_seam_reads.py::test_the_criteria_reaches_an_entity_that_is_only_joined` - Tree: `7a14f79e6912bd4fd9ffaafd022044afcf301bc3` - Lane task: 7161 - Flake record: 01a15016-3282-728d-8da7-6b79b28b4f62 Make it deterministic; do not delete it unless it tests nothing.

Fixed the flaky join-only seam test: it now captures SQL on the session's own engine, because _LanedSession binds background_engine when the thread holds a request connection. Reproduced red before the fix, green after; quick gate green. PR #5855 merged (fdbef1d); deploy success.