Flaky static exec integration test closes broker association during teardown

[wdcui]

Summary

The Ubuntu Build and Test job for #1411 intermittently failed in litebox_runner_linux_userland::run::test_static_exec_with_rewriter because the runner closed the active broker association. The unchanged commit passed local reproductions and a rerun of the same workflow, suggesting a runner/broker association teardown race rather than a deterministic regression.

Failure evidence

• Commit: c59bb54c1c82addc48a8a9b32c3a9871a35bab54
• Workflow run: https://github.com/microsoft/litebox/actions/runs/35958317469
• Failed job, attempt 1: https://github.com/microsoft/litebox/actions/runs/35958317469/job/107501258626
• Runner: ubuntu-latest
• Failed step: Run ./.github/tools/github_actions_run_cargo nextest
• Result: 977 tests run: 976 passed, 1 failed, 22 skipped
• Failing test: litebox_runner_linux_userland::run::test_static_exec_with_rewriter
• The test exhausted all three nextest attempts.
• Reported error: BrokenPipe: runner closed the active broker association

The failure surfaced during different inner C programs across retries. The first attempt reached thread_exit_exec_rewriter; later attempts failed around execve_exec_rewriter. During an exec retry, the guest printed:

[OK] exec caller became leader; cloexec fd 3 closed; keep fd 4 open

The broker command nevertheless exited with status 1 afterward, which points toward association shutdown/teardown rather than the guest validation itself.

Reproduction attempts

The failure did not reproduce locally on the same commit:

cargo nextest run -p litebox_runner_linux_userland --test run test_static_exec_with_rewriter --no-fail-fast
PASS in 24.317s

cargo nextest run -p litebox_runner_linux_userland --test run --no-fail-fast
15/15 passed; affected test passed in 12.993s

cargo nextest run --no-fail-fast
977/977 passed; affected test passed in 13.353s

Successful unchanged rerun

The failed GitHub Actions job was rerun without any code changes:

• Successful job, attempt 2: https://github.com/microsoft/litebox/actions/runs/35958317469/job/107629199846
• Same commit: c59bb54c1c82addc48a8a9b32c3a9871a35bab54
• The nextest step and all PR checks passed.

We should track the runner/broker association teardown path and improve diagnostics so a future occurrence identifies which side closes the channel and why.