[FrankYang0529]

Why

• DagRun._emit_true_scheduling_delay_stats_for_finished_state() still builds dagrun.<dag_id>.first_task_scheduling_delay instead of leaving it to the stats.timing("dagrun.first_task_scheduling_delay", ...) call on the next line, which already emits that name.

How

• Delete the stats.timing(f"dagrun.{dag.dag_id}.first_task_scheduling_delay", ...) call and let the registry supply the legacy name.

Verification

• uv run --frozen --project airflow-core pytest airflow-core/tests/unit/models/test_dagrun.py

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Was generative AI tooling used to co-author this PR?

• Yes - Claude Code

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