test: reduce CI flakiness in Ray-backed tests

[Copilot]

Summary

Tighten test reliability for failures seen on main: intermittent Ray worker/actor startup crashes in unit tests. The changes target test isolation and narrow retry handling without weakening the underlying assertions.

Changes

• ZMQ test fixture isolation

  • Replace per-test os.environ mutation for PLUGBOARD_FLAGS_ZMQ_PUBSUB_PROXY with scoped DI settings overrides.
  • Apply the change consistently across shared, unit, and integration ZMQ fixtures to avoid cross-test/process races when Ray workers start in parallel.

• Targeted reruns for Ray-backed flakes

  • Add limited reruns to the Ray-backed unit tests that intermittently fail during actor/worker startup:
    • test_multiprocessing_channel
    • test_state_backend_init
    • test_state_backend_init_with_existing_job
  • Keep reruns narrow to preserve signal and avoid broad pipeline inflation.

testing_settings = Settings.model_validate({"flags": {"zmq_pubsub_proxy": zmq_pubsub_proxy}})
with override_settings(testing_settings):
    yield ZMQConnector

[toby-coleman]

@copilot Remove the changes to the smoke tests. They are not that flaky.

[Copilot]

@copilot Remove the changes to the smoke tests. They are not that flaky.

Removed the smoke-test changes in 3d8471b.

[github-actions]

Benchmark comparison for 07750b3c (base) vs 3d8471bb (PR)

------------------------------------------------------------------------------------------------------------------ benchmark: 2 tests -----------------------------------------------------------------------------------------------------------------
Name (time in ms)                                                                         Min                 Max                Mean            StdDev              Median               IQR            Outliers     OPS            Rounds  Iterations
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
test_benchmark_process_run (main/.benchmarks/Linux-CPython-3.14-64bit/0001_base)     352.8164 (1.0)      358.8909 (1.0)      355.8906 (1.0)      2.1652 (1.0)      356.0099 (1.00)     2.0833 (1.0)           2;0  2.8099 (1.0)           5           1
test_benchmark_process_run (pr/.benchmarks/Linux-CPython-3.14-64bit/0001_pr)         353.6085 (1.00)     364.1089 (1.01)     357.5500 (1.00)     4.7998 (2.22)     354.4052 (1.0)      7.7744 (3.73)          1;0  2.7968 (1.00)          5           1
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Legend:
  Outliers: 1 Standard Deviation from Mean; 1.5 IQR (InterQuartile Range) from 1st Quartile and 3rd Quartile.
  OPS: Operations Per Second, computed as 1 / Mean