fix(db_cleanup): handle NULL start_date in dag_run cleanup

[bileyroy]

closes: #69872

Summary

When start_date is NULL (e.g. dag_runs that were marked failed before execution began), the cleanup query's WHERE start_date < cutoff evaluates to NULL and silently skips these rows forever.

Changes

• Add fallback_recency_column_name field to _TableConfig (set to created_at for dag_run)
• Add --fallback-cleanup-on-null CLI flag (opt-in, off by default)
• When enabled, apply COALESCE(start_date, created_at) to:
  • Main WHERE condition: so NULL start_date rows are found
  • keep_last subquery (MAX): so the latest run is correctly identified via fallback
  • keep_last JOIN condition: so the latest run is properly protected
• Add 5 unit tests covering flag behavior and keep_last protection

Test plan

• pytest airflow-core/tests/unit/utils/test_db_cleanup.py -xvs — all passed
• ruff check / ruff format — clean
• mypy — clean
• Manual testing via airflow standalone

Testing with manual runs (keep_last not applicable):

Testing with backfill runs (keep_last applicable)

All 3 backfill runs have start_date = NULL. With --fallback-cleanup-on-null, only 2 rows are returned for deletion, the newest run is correctly protected by keep_last via COALESCE(start_date, created_at):

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