Ignore task start_date and end_date outside scheduled runs

[yuseok89]

closes: #38513
related: #71758

A Dag run whose logical date falls outside a task's start_date / end_date window creates no task instance for that task. When every task is filtered out this way, the run has no task instances and is immediately reported as success without anything having run.

Backfill runs were already exempt from this window. Every run type except scheduled runs is now exempt too, so the window only limits what the scheduler creates.

Improvements over #71758

• Ignore task start_date and end_date on manual runs #71758 only exempted manual runs. Runs created by TriggerDagRunOperator (operator_triggered) and asset materialization runs still ended up empty, so they are covered now. The condition is run_type != SCHEDULED, which also covers run types added later.
• The FAQ has a dedicated entry on which runs the dates apply to. It covers a task whose end_date has passed, which now runs in manual runs, and the depends_on_past=True with catchup=False case, which needs ignore_first_depends_on_past=True.

Verified in the UI

A Dag with a start_date of 2050 and three chained @task functions (extract → transform → load).

Before

Screen.Recording.2026-08-18.at.11.19.52.PM.mov

After

Screen.Recording.2026-08-18.at.11.24.09.PM.mov

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

• Yes (please specify the tool below)
  • Claude Code (Opus 5.5)

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