Fix flaky test tests/test_git_mirror_isolation.py::test_a_planted_key_steers_no_daemon_git[create_worktree-local-remote]

The merge lane saw `tests/test_git_mirror_isolation.py::test_a_planted_key_steers_no_daemon_git[create_worktree-local-remote]` 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-local-remote]` - Tree: `97f5c44e1a99d35c0c115ffec4f12ddbf7381add` - Lane task: 7584 - Flake record: 01a151b7-a0b7-7aef-990c-c8c37f1b3d43 Make it deterministic; do not delete it unless it tests nothing.

Root cause: leaked FakeBroker accept threads in conftest stole the mirror test's TLS connections through fd reuse. Task 7609 (ce2252ce2) already fixed it on develop. Verified: 705 passed, 0 stale threads. No PR needed.