Fix backfill validate n plus one

[ColtenOuO]

Summary

A new helper _fetch_latest_dag_runs_for_infos pre-loads all DagRuns the loop will need into a dict, replacing the per-info SELECT inside the loop with an in-memory lookup. This collapses N database round-trips into one batched SELECT plus N dict reads.

Per @jason810496's review on #66222, a regression test (test_create_backfill_does_not_re_query_each_info) uses assert_queries_count to lock in the post-fix query count.

Impact

• 20-day backfill: 215 → 195 round-trips (one saved per DagRunInfo).

Verification

• Full backfill suite: 60 passed in airflow-core/tests/unit/models/test_backfill.py.
• API-layer suite: 39 passed in airflow-core/tests/unit/api_fastapi/core_api/routes/public/test_backfills.py.

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

• Yes (Claude Code)

Generated-by: Claude Code following the guidelines

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Important

🛠️ Maintainer triage note for @ColtenOuO · by @potiuk · 2026-06-18 13:57 UTC

Paused pending your next update — this PR has been inactive for ~43 days, so it's been moved to draft to keep the review queue clear:

• Rebase on the latest main, address any new failures, and mark it Ready for review when you pick it back up — no rush.
• See the Pull Request quality criteria.

The ball is in your court — you've been assigned to this PR.

_{Automated triage — may be imperfect; a maintainer takes the next look.}