fix(tests): keep MPS sockets within macOS path limit

[TATP-233]

Summary

• Added a shared test fixture that creates Unix-domain sockets under a short /tmp-rooted temporary directory.
• Updated CUDA MPS and off-policy runner tests to use that fixture.
• Production MPS validation and training behavior are unchanged; this only fixes macOS tests that exceeded the AF_UNIX path limit.

Linked Work

• Issue: macOS MPS tests exceed AF_UNIX path limit #2089
• Parent roadmap (when applicable): none
• Roadmap declared base (when applicable): none
• Milestone: none
• Base branch: main

Validation

• make test-all passed on the final local head before this PR was created or updated
• Additional task-specific validation listed below

Commands actually run:

uv run pytest tests/algos/test_offpolicy_double_buffer_runner.py::test_cuda_process_sharing_request_fails_before_env_materialization tests/training/test_cuda_process_sharing.py::test_valid_single_rank_request_records_server_evidence tests/training/test_cuda_process_sharing.py::test_valid_identity_compares_physical_uuid_not_ordinal tests/training/test_cuda_process_sharing.py::test_single_host_dp_rank_records_rank_scoped_evidence tests/training/test_cuda_process_sharing.py::test_gpu_identity_mismatch_rejects tests/training/test_cuda_process_sharing.py::test_unreachable_daemon_rejects_without_fallback -q
uv run pytest tests/training/test_cuda_process_sharing.py tests/algos/test_offpolicy_double_buffer_runner.py -q
uv run ruff format --check tests/conftest.py tests/training/test_cuda_process_sharing.py tests/algos/test_offpolicy_double_buffer_runner.py
uv run ruff check tests/conftest.py tests/training/test_cuda_process_sharing.py tests/algos/test_offpolicy_double_buffer_runner.py
make check
make test
make test-all

Remote CI route:

• Base main: CI run 37785612681 passed for commit abcb74209680bc02367d3be1f4dd4870392acec9 (all 6 jobs).

Impact

• Backend impact: none
• Platform impact: macOS
• Training effect expected: no

Artifacts

• W&B: none
• benchmark result: none
• video / screenshot: none
• ONNX / checkpoint: none

Checklist

• Added or updated tests where needed
• Updated docs if behavior or workflow changed
• Linked the driving issue
• Noted any follow-up work explicitly