Stop task supervisors after an init process reaps its child

[hooiv]

What type of change is this?

Bug fix.

What does this PR do?

Fixes a task supervisor hang when an init process such as dumb-init reaps the task child before the supervisor collects its exit status.

psutil.Process.wait(timeout=0) raises NoSuchProcess in that situation. Previously, the supervisor left both _exit_code and _process_exit_monotonic unset, so the socket cleanup timeout never started and the supervisor could remain alive indefinitely while retaining task sockets and memory.

The supervisor now treats the child as exited with the same unknown exit code used by the existing already-terminated kill path and starts the cleanup timer. Added a regression test for the reaped-child path.

Fixes #70117.

Testing

• uv run --project task-sdk ruff format task-sdk/src/airflow/sdk/execution_time/supervisor.py task-sdk/tests/task_sdk/execution_time/test_supervisor.py
• uv run --project task-sdk ruff check --fix task-sdk/src/airflow/sdk/execution_time/supervisor.py task-sdk/tests/task_sdk/execution_time/test_supervisor.py
• Focused Linux test: TestWatchedSubprocess::test_process_already_reaped — passed
• Linux test_supervisor.py: 166 passed, 6 skipped; remaining failures/errors were pre-existing mounted-Windows checkout issues (path-sensitive log assertions, missing optional provider/shared-package setup)

Checklist

• The change includes regression coverage.
• The commit has a Signed-off-by line.
• Full CI verification

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