AIP-97 (1/3): add failure_kind to the task-failure listener + infra_reason column

[1fanwang]

Part 1 of 3 of the AIP-97 stack. Consolidated view with the whole design and e2e: apache/airflow#66405.

Why

on_task_instance_failed sees only the worker-side error. When a task is killed from outside, such as a pod eviction, OOM kill, force-deleted pod, or lost heartbeat, the worker never raises. The listener gets no cause. An eviction and a real ValueError look identical to alerting and lineage.

What

Foundation only, with no executor wiring or retry change yet:

• on_task_instance_failed gains an optional failure_kind enum: infra, application, timeout, or manual. pluggy matches by name, so a listener that omits it keeps working.
• The worker sets application or timeout; a manual mark-failed sets manual. infra arrives with the executor bridge (2/3).
• A new nullable infra_reason column on task_instance holds the executor's reason token.

Testing

test_listener_types: a listener declaring failure_kind receives it, and one that omits it still fires. airflow db migrate creates the infra_reason column.

Stack

1. this PR: failure_kind plus infra_reason
2. Kubernetes classifier
3. opt-in refund

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