Refresh the shared container URL after a restart

[robinskil]

Problem

test_persistence.py::test_catalog_survives_restart restarts the shared container. Docker then maps a new host port.

The test sets the new URL on its own client only. The session fixture beacon_container keeps the old port.

test_rbac.py builds a client from that stale URL. Every request gets Connection refused.

Evidence

• pytest test_rbac.py alone: 6 passed, 1 skipped.
• pytest test_persistence.py test_rbac.py: 1 failed, 5 errors.

Failed tests

• test_rbac.py::test_list_flags_super_user_and_anonymous
• Five more tests in test_rbac.py error at setup.

Task

1. Make beacon_container return a mutable object, not a string.
2. Update that object after the restart.
3. Build every client from the current value.
4. Run both files together again.

Environment

Branch features/reimplement-atlas. Image beacon-integration:local.