Task supervisor lingers indefinitely on CeleryExecutor when psutil.Process.wait() returns None (socket_cleanup_timeout never triggers)

### Under which category would you file this issue? Task SDK ### Apache Airflow version 3.1.8 ### What happened and how to reproduce it? Task supervisor processes (`airflow worker -- <uuid>`) remain alive indefinitely after their task subprocess has finished and exited. The `workers.socket_cleanup_timeout` safety-net (introduced in 3.0.2 via https://github.com/apache/airflow/pull/51180) never triggers because the supervisor never detects the child process exit — `_process_exit_monotonic` is never set. On our production Celery workers running for ~3 days: - **worker-0:** 26 stuck supervisors, 357 zombie processes, 0 running tasks - **worker-1:** 30 stuck supervisors, 417 zombie processes, 0 running tasks Each stuck supervisor holds ~340 MB of RSS. With 26–30 orphaned supervisors, this accounts for ~8–10 GB of wasted memory per worker pod, causing OOM kills on containers with 6–8 Gi limits. ## How to reproduce 1. Deploy Airflow 3.1.8 with CeleryExecutor using `dumb-init` as container entrypoint (standard Helm chart setup): ``` command: ["/usr/bin/dumb-init", "--"] args: ["bash", "-c", "exec airflow celery worker -H ${HOSTNAME}"] ``` 2. Configure `worker_concurrency=16` (or any value) 3. Run a DAG with dynamically mapped tasks (using `expand()`), e.g. a task that produces many map instances 4. Wait for tasks to complete 5. Check for lingering supervisors: ```bash # Inside the worker container: count=0 for pid in /proc/[0-9]*; do cmd=$(tr '\0' ' ' < "$pid/cmdline" 2>/dev/null) case "$cmd" in "airflow worker -- "*) count=$((count+1));; esac done echo "live supervisors: $count" ``` 6. Expected: 0 supervisors when no tasks are running 7. Actual: supervisors accumulate over time, never exit ### What you think should happen instead? ## Root cause analysis ### Environment - **Airflow:** 3.1.8 - **Executor:** CeleryExecutor with `worker_concurrency=16` - **Container init:** `dumb-init` (PID 1) — standard in Airflow Helm chart - **Work…