fix: restore Airflow 2 Task Instances view semantics for mark-success

[anmolxlight]

Description

Fixes #67707

In Airflow 2, marking a task as success from the Task Instances view (Browse → Task Instances) used ti.set_state() directly — it only changed the selected task instance's state and did not clear or resume downstream tasks that were in upstream_failed or failed state.

Airflow 3's dag.set_task_instance_state() unconditionally cleared downstream failed/upstream_failed tasks after setting state, even when downstream=False was passed. This broke backward compatibility for users migrating from Airflow 2 who relied on the Task Instances view preserving downstream task state.

Changes

This change gates the downstream clearing behavior behind the downstream parameter in both set_task_instance_state() and set_task_group_state():

• downstream=False (default): Only the target TI state is changed — no downstream clearing. This restores the Airflow 2 Task Instances view semantics.
• downstream=True: Downstream failed/upstream_failed tasks are also cleared and can resume. This preserves the existing Airflow 3 DAG/Grid view behavior.

Files changed

• airflow-core/src/airflow/serialization/definitions/dag.py:
  • set_task_instance_state(): Wrap downstream clearing in if downstream:
  • set_task_group_state(): Same treatment for consistency
• airflow-core/tests/unit/models/test_dag.py:
  • test_set_task_instance_state: Updated to expect no clearing with downstream=False (default)
  • New test_set_task_instance_state_downstream_clears_failed: Verifies clearing with downstream=True
  • test_set_task_instance_state_mapped: Updated to pass downstream=True since it expects clearing

Backward compatibility

• Users who mark task instances as success without selecting "downstream" in the dialog get the old Airflow 2 behavior (no downstream clearing).
• Users who explicitly select "downstream" get the downstream clearing behavior.
• The DAG/Grid view's existing behavior is preserved.

Checklist

• Code changes
• Tests updated
• No new imports required

[Pedrinhonitz]

Please ensure that all quality tests are successful.

• Tests (AMD) / CI image checks / Static checks (pull_request)
• Tests (AMD) / Low dep tests:core / All-core:LowestDeps:14:3.10:API...CLI (pull_request)
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