Add on_task_instance_skipped listener hookspec

[tstrilka]

Summary

This PR adds a new listener hookspec on_task_instance_skipped that is called when a task raises AirflowSkipException.

Related issue: closes #59370

Motivation

The current listener interface provides hooks for:

• on_task_instance_running - when a task starts
• on_task_instance_success - when a task succeeds
• on_task_instance_failed - when a task fails

But there is no hook for when a task is skipped via AirflowSkipException. This gap prevents listener implementations from handling self-skipping tasks.

Scope & Limitations

Important: This hook is called only when a task raises AirflowSkipException during execution — i.e., when a task has actually started and then skips itself.

This hook does NOT cover tasks skipped by:

• Trigger rules (e.g., upstream failures)
• BranchPythonOperator (tasks not in the selected branch)
• ShortCircuitOperator
• Manual state changes via UI/API
• Scheduler-level decisions

For comprehensive skip tracking across all skip types, use DAG-level events (on_dag_run_success/on_dag_run_failed) which include AirflowStateRunFacet with the complete tasksState map.

Changes

File	Description
airflow-core/src/airflow/listeners/spec/taskinstance.py	Added on_task_instance_skipped hookspec
task-sdk/src/airflow/sdk/execution_time/task_runner.py	Call listener in finalize() when state is SKIPPED
task-sdk/tests/task_sdk/execution_time/test_task_runner.py	Added test for skipped listener

Follow-up

OpenLineage provider implementation will be submitted in a separate PR after this is merged.

Testing

• Added unit test test_task_runner_calls_listeners_skipped that verifies the listener is called when a task raises AirflowSkipException

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[kacpermuda]

Thanks, this will definitely help listeners to correctly "see" how the task finished it's execution, after they receive an information about task start. Left a few suggestions to improve the docstring.

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.