Fix flaky test tests/token_billing/test_metering.py::test_a_session_is_metered_for_an_hour_after_it_ends_and_refused_after

The merge lane saw `tests/token_billing/test_metering.py::test_a_session_is_metered_for_an_hour_after_it_ends_and_refused_after` fail and then pass on a rerun of the same tree. - Test: `tests/token_billing/test_metering.py::test_a_session_is_metered_for_an_hour_after_it_ends_and_refused_after` - Tree: `336500b9600a16cbb307ba31a69997424d5b7820` - Lane task: 9614 - Flake record: 01a16cd3-cecc-706d-a186-051d9c058bb2 Make it deterministic; do not delete it unless it tests nothing. - Also flaky, same file: `tests/token_billing/test_metering.py::test_a_malformed_post_is_refused_whole[calls0]` (tree `336500b9600a16cbb307ba31a69997424d5b7820`, lane task 9614, flake record 01a16cd3-cf76-72f9-8f91-fb4fb4448d6e) - Also flaky, same file: `tests/token_billing/test_metering.py::test_a_malformed_post_is_refused_whole[calls3]` (tree `336500b9600a16cbb307ba31a69997424d5b7820`, lane task 9614, flake record 01a16cd3-d020-78b2-82cc-19a55e6af2cf) - Also flaky, same file: `tests/token_billing/test_metering.py::test_a_call_on_an_unpriced_model_is_never_billed_and_is_noted_for_the_operator` (tree `336500b9600a16cbb307ba31a69997424d5b7820`, lane task 9614, flake record 01a16cd3-d0c6-7d21-903f-29bc91b42af6) - Also flaky, same file: `tests/token_billing/test_metering.py::test_a_node_report_after_aws_billed_the_call_joins_it` (tree `336500b9600a16cbb307ba31a69997424d5b7820`, lane task 9614, flake record 01a16cd3-d390-71d1-b501-f2c9f333e23d)

Root cause: test_tenant_receive patched global time.monotonic=inf, killing gw1's TestClient portals (all 60 lane flakes incl. 5 metering). Now patches receive's own clock. Quick gate green, 58 targeted pass; PR #6802 queued. Memory: proposed 1 gotcha.