Fix KubernetesPodOperator succeeding after SIGTERM/on_kill interrupt

[sankalpsthakur]

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When KubernetesExecutor task pods and KubernetesPodOperator child pods are interrupted together (spot drain / dual SIGTERM), the task instance was finalized as SUCCESS even though the child never finished.

Root cause: task_runner SIGTERM handling calls on_kill(), which sets _killed and deletes the child. cleanup() then returned immediately so execute() completed normally. The supervisor records SUCCESS for a 0-exit with no terminal-state message. Interrupting only the child still failed (404 / PodFailed) because on_kill() never ran.

This does not revert #69034 (warm-shutdown supervisor). SIGTERM on the supervisor is still not treated as task failure. The change is KPO-only: after on_kill(), cleanup still skips a second delete (avoids 404 retries) but raises AirflowException so the TI is failed / up_for_retry.

Summary

• KubernetesPodOperator.cleanup() raises if on_kill() already ran, instead of returning as success.
• Unit tests cover the _killed cleanup path and the execute_sync race (SIGTERM/on_kill during wait, then execute continues).

Validation

• Published apache-airflow-providers-cncf-kubernetes==10.21.0: cleanup() after _killed=True returned None (the SUCCESS bug).
• Same call against this change: AirflowException: Pod sleep-worker was interrupted before it completed.
• Tests added:
  • test_cleanup_raises_when_on_kill_already_ran
  • test_execute_sync_on_kill_during_wait_is_not_success
• Full test_pod.py file is db_test (breeze). Local proof used Airflow 3.1.3 + overlaid operator module, no live cluster.

Fixes #71202

AI/LLM disclosure

• AI coding tools (including Grok and/or Codex agent-assisted editing) were used to help draft or modify code and this PR description.
• I reviewed the complete change, understand the reasoning, and ran the reported local tests before submitting.
• This submission is original work of authorship under the project CLA / contributor terms; AI output was not pasted unreviewed.

---

Was generative AI tooling used to co-author this PR?

• Yes (Grok / Cursor agent)

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[sankalpsthakur]

Rebased onto main and used PodInterrupted (plus ruff-format) so static checks should pass.