fix(tests): poll for runner exit instead of single-step assert (FILTER-461)

[stwilt]

📋 What does this PR do?

Adds a wait_for_runner_exit(runner, timeout=5.0) helper in tests/test_filter.py that polls runner.step() until it returns something other than False (or times out), and replaces five fragile single-step shutdown assertions across four TestFilterOld tests:

• test_topo_balance_step
• test_topo_ephemeral_join_step
• test_topo_doubly_ephemeral_tee_step (two callsites)
• test_topo_subscribe_wildcard_all_step

Each callsite previously asserted that a single runner.step() immediately after qout.put(None) would return the exit-codes list ([0] * N for N filters). With the helper, the assertion now waits up to 5 s for the runner to actually report termination — preserving the original assertion semantics, but with the same step → settle → assert discipline the data-frame iterations already use.

🔍 Why is this needed?

test_topo_balance_step started failing on Python 3.10 only as of Create Release dry-run run #25936326919, while passing on 3.11 / 3.12 / 3.13. The same test passed on 3.10 for the v0.1.30 release (run #24800470941, 2026-04-22). Between those, #82 introduced the zero-copy IPC transport with SHM — a perf rewrite of the layer this test exercises. The faster IPC shifted timing margins such that on 3.10's GIL/scheduling, at least one of N filters routinely hasn't yet drained the None shutdown sentinel + reported its exit code by the time the next step() runs. Result: step() returns False ("not done") instead of [0, 0, …].

This is test fragility, not a runtime regression — the 9 data-frame iterations in the same test exercise the full pipeline correctly across 3.10 too. But it was also a release blocker: the Create Release workflow gates on every matrix Python version passing run-tests, so any release dispatch was rolling dice on the 3.10 job.

See FILTER-461 for the full root-cause analysis.

🧪 How was it tested?

Set up a fresh Python 3.10 venv with the project in editable mode (uv venv .venv-310 --python 3.10, uv pip install -e ".[all]", install pytest), and ran the failing test:

$ .venv-310/bin/pytest tests/test_filter.py::TestFilterOld::test_topo_balance_step -v
…
tests/test_filter.py::TestFilterOld::test_topo_balance_step PASSED  [100%]
1 passed in 3.66s

Repeated 3× consecutively — all green. Also ran all four patched tests together — all pass:

tests/test_filter.py::TestFilterOld::test_topo_balance_step PASSED  [ 25%]
tests/test_filter.py::TestFilterOld::test_topo_ephemeral_join_step PASSED  [ 50%]
tests/test_filter.py::TestFilterOld::test_topo_doubly_ephemeral_tee_step PASSED  [ 75%]
tests/test_filter.py::TestFilterOld::test_topo_subscribe_wildcard_all_step PASSED  [100%]
4 passed in 8.72s

Final CI confirmation will come from the Create Release re-dispatch after this merges — all four Python matrix versions should pass run-tests consistently.

🔗 Related Issues

• FILTER-461 — root-cause analysis and fix proposal (this PR implements the proposed fix).
• Unblocks the v0.2.1 release ceremony (validation of the PLAINSIGHT_PYPI_TOKEN publish path was caught by this flake before reaching the PyPI upload step).

✅ Checklist

• I have read and agreed to the terms of the LICENSE
• I have read the CONTRIBUTING guide
• I have followed the coding style
• I have signed all commits in compliance with the DCO (git commit -s)
• I have added or updated tests as needed (the fix IS the test change; helper is documented in-line)
• I have added or updated documentation as needed (helper has a docstring explaining the FILTER-461 context)

[lucasmundim]

LGTM — the polling helper is sound and the call-site replacements are correct. Two non-blocking nits below.