Fix flaky test tests/test_git_mirror_isolation.py::test_a_planted_key_steers_no_daemon_git[create_worktree-xdg-code]

The merge lane saw `tests/test_git_mirror_isolation.py::test_a_planted_key_steers_no_daemon_git[create_worktree-xdg-code]` fail and then pass on a rerun of the same tree. - Test: `tests/test_git_mirror_isolation.py::test_a_planted_key_steers_no_daemon_git[create_worktree-xdg-code]` - Tree: `97f5c44e1a99d35c0c115ffec4f12ddbf7381add` - Lane task: 7584 - Flake record: 01a151b7-92ba-75d2-b819-adf8c841b242 Make it deterministic; do not delete it unless it tests nothing.

Root cause: a FakeBroker serve thread leaked by an earlier test stole the isolation test's TLS connections. Same fix already on develop (ce2252ce2); repro failures went from 46 to 0. No PR. Drafts #7666 and #7667 filed for unrelated container failures.