Cover clearing task state based on dag run clear #72929

Why?

#72100 added discard-by-default behavior for task_state_store entries when a task instance
is cleared via the POST /dags/{dag_id}/clearTaskInstances endpoint — checkpoints are
discarded unless keep_task_state is set, so a cleared task starts over instead of silently
resuming from a stale checkpoint after the underlying code/data was fixed.

That fix only applies to that one endpoint. Every other way of clearing a task instance still
keeps task state unconditionally, because each of them calls clear_task_instances() directly
instead of going through the new discard logic:

• Clearing an entire Dag run (the "Clear Run" dialog / API) —
  perform_clear_dag_run -> dag.clear() -> clear_task_instances().
• Marking a task as failed/success, which clears downstream tasks as a side effect.
• airflow dags clear / airflow tasks clear (core CLI) —
  cli/commands/dag_command.py -> clear_task_instances() directly.

Follow-up work

Thread keep_task_state (or an equivalent opt-out) through clear_task_instances() itself, so
every caller (REST endpoint, Dag-run clear, CLI, mark-as-success/failed) converges on the same
discard-by-default behavior. This likely needs:

• A keep_task_state parameter on clear_task_instances().
• Plumbing it through dag.clear(), perform_clear_dag_run, and the core CLI's
  airflow dags clear / airflow tasks clear.
• Deciding the right default/UX for callers that have no request body to carry the flag today
  (e.g. CLI defaults, mark-as-success/failed).

Acceptance criteria

• Clearing a Dag run discards task_state_store entries by default, matching the task-instance
  clear endpoint, with an equivalent opt-out to keep state.
• airflow dags clear / airflow tasks clear behave consistently with the REST endpoints.
• Docs (resumable-tasks.rst) no longer need to caveat which clear surface actually discards.