Add auto-clear of failed child tasks to TriggerDagRunOperator (#53402)

[eugeneo17]

closes: #53402

TriggerDagRunOperator gains an opt-in auto_clear_failed_tasks flag (default False) that lets a parent-task retry resume a failed child run from its point of failure instead of dead-ending.

Motivation
Chained-DAG pipelines commonly pin the child's trigger_run_id (or its logical_date) so the child run is idempotent, one run per business date/key, so a parent retry doesn't spawn duplicate child runs. But with wait_for_completion=True, that same stability makes retries a dead end: once the pinned child run has failed, re-running the parent task raises DagRunAlreadyExists, and the only escape today is reset_dag_run=True, which wipes the whole child and re-runs tasks that already succeeded. There is no way to resume a failed child from where it stopped without manual cleanup.

What this does
On the parent task's retry, when the pinned child run already exists in a terminal failed state, auto_clear_failed_tasks=True clears only the failed tasks (and their downstream) of that run and re-runs them, preserving already-successful tasks - the child resumes rather than restarts or errors.

The failed-only clear is delivered server-side via a new Execution API version (2026-11-13, AddOnlyFailedToClearDagRunPayload); the only_failed flag is threaded operator → task-runner → supervisor → SDK client, and on a 409 conflict the client routes the trigger to the /clear endpoint with only_failed=true.
reset_dag_run keeps precedence: if both are set the whole run is cleared, and at most one clear ever happens.
It is version-gated, against a core older than 2026-11-13 the operator raises NotImplementedError rather than silently downgrading to a whole-run clear.

Scope and notes
Synchronous only (wait_for_completion=True); not supported with deferrable=True (setting both raises ValueError).
Because previously-successful child tasks are not re-run and failed ones are, the triggered DAG's tasks should be idempotent - called out in the operator docstring and the operator guide.
A healed child run's final state reads success, so run-level alerting won't see the transient failure (per-attempt logs are preserved). Emitting a metric/event when a heal fires would close that gap and is proposed as a follow-up rather than part of this PR.

Testing
Unit tests for the operator, the Task SDK (comms/supervisor/task-runner/client), and the Execution API version + payload. Includes a regression test asserting the failed-only clear is reached when reset_dag_run=False (the auto_clear path), which fails against a client that gates the clear on reset_dag_run alone.

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[eugeneo17]

Hi, this PR implements #53402: an opt-in auto_clear_failed_tasks flag for TriggerDagRunOperator (default False), so a parent retry can resume a failed child run instead of dead-ending. I've just merged the latest main (no conflicts). It's my first contribution here, so the CI workflows need approval to run. Could a committer approve them and take a look when there's time? Happy to adjust anything.