tests: rethrow worker-thread assertion in portable runner test to fix assert_that flake (fixes #35211)

[jh1231223]

Thanks for reviewing!

It seems the current failures are unrelated to the modifications in this PR. If I’ve overlooked something, please let me know and I’ll address it promptly.

Context / Issue

• Addresses flaky behavior tracked in [Failing Test]: :sdks:python:test-suites:tox:py39:testPython39 can be flaky #35211.
• In PortableRunnerTest::test_assert_that, a failing assert_that raised on a worker thread was only surfaced as a PytestUnhandledThreadExceptionWarning, so the test could pass intermittently.

What this change does

• Captures threading.excepthook during the test run.
• Wraps beam.Pipeline.run(...).wait_until_finish() so that any captured worker-thread exception is rethrown on the main thread at the end of the run.
• Updates test_assert_that to use this shim and assert the expected failure via assertRaisesRegex, turning the flaky pass into a deterministic failure for the negative case.

Why
Ensures the failure path for assert_that is exercised reliably and reported as a proper test failure, rather than a warning emitted from a background thread.

Verification
Stress run x200 (no flakes):

for i in {1..200}; do
  echo "Run $i"
  pytest -q apache_beam/runners/portability/portable_runner_test.py::PortableRunnerTest::test_assert_that \
    -W error::pytest.PytestUnhandledThreadExceptionWarning -s --maxfail=1 || break
done

Result: 200/200 passes; no thread-exception warnings.

[gemini-code-assist]

Summary of Changes

Hello @jh1231223, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical flakiness issue in a specific test by implementing a robust exception propagation mechanism. It ensures that exceptions originating from worker threads during pipeline execution are consistently re-raised on the main thread, thereby converting an intermittently passing test (due to warnings) into a reliably failing one when an assertion condition is not met. This significantly improves the stability and correctness of the test suite.

Highlights

• Flaky Test Fix: Addresses a known flaky behavior in PortableRunnerTest::test_assert_that where assert_that failures in worker threads were only surfaced as warnings, leading to intermittent test passes.
• Exception Handling Enhancement: Introduces a mechanism to capture threading.excepthook during test runs, allowing exceptions raised on worker threads to be caught.
• Main Thread Exception Rethrowing: Wraps beam.Pipeline.run(...).wait_until_finish() to rethrow any captured worker-thread exceptions on the main thread at the end of the pipeline run.
• Deterministic Test Failure: Updates test_assert_that to utilize this new exception handling shim, transforming the previously flaky test into a deterministic failure for the negative case, ensuring reliable testing of assertion failures.

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