Make cancelled-JoinError test fixture deterministic

[leynos]

Closes #139.

Summary

The cargo-mutants unmutated baseline intermittently failed on
supervisor::tests::logs_failures::case_3 (run 29074661946) while plain CI
stayed green. The defect is not the log-capture path — the test already uses a
scoped with_default subscriber — but a race in the test fixture that builds
the cancelled JoinError.

create_cancelled_join_error spawned a task that yields once and then
completes, on a multi-threaded runtime, and aborted it afterwards. The spawned
task runs concurrently on a worker thread, so under load it can finish before
abort() lands; handle.await then returns Ok(()) and unwrap_err()
panics (called Result::unwrap_err() on an Ok value). Machine load explains
the execution-shape dependence: cargo-mutants' baseline runs on a busier
machine profile than the plain make test job.

Fix

Abort a task awaiting std::future::pending::<()>() on a current-thread
runtime. The future can never complete, and on a current-thread runtime the
task cannot even be polled before block_on regains control, so the abort
always produces the cancelled JoinError. Test semantics are unchanged: the
fixture still hands log_task_failure a genuine cancelled JoinError.

Review walkthrough

• crates/comenqd/src/supervisor/tests.rs
  — create_cancelled_join_error now uses a current-thread runtime and a
  never-completing future; the doc comment records why.

Validation

• Reproduced the failure locally before the fix: running the comenqd test
  binary's logs_failures filter repeatedly failed with the exact panic at
  tests.rs:36 (unwrap_err() on Ok) roughly once per 100 runs.
• After the fix: 500 consecutive runs of the same filter pass.
• cargo test --workspace --all-features (the cargo-mutants baseline shape,
  with the caller's extra-args) passed three consecutive full runs.
• make check-fmt and make lint (Clippy; the Whitaker Dylint suite runs in
  CI) pass.

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 98780d23-c153-4d8c-a84f-f37c796a42c4

📥 Commits

Reviewing files that changed from the base of the PR and between 85ffd50 and fcd84d5.

📒 Files selected for processing (1)

• crates/comenqd/src/supervisor/tests.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-mutation-baseline

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