Adds a new public API endpoint for backfill dag run entries - ` GET /backfills/{backfill_id}/dag_runs

[shivaam]

Adds a new public API endpoint GET /backfills/{backfill_id}/dag_runs that returns
the BackfillDagRun entries for a given backfill with joined DagRun state. Users
can see what happened in a backfill: which dates ran, their states, and which slots
were skipped (with reason).

Previous PR: #64089 explored enriching BackfillResponse directly. Based on
feedback from @uranusjr, this PR takes the cleaner approach of a dedicated
sub-resource endpoint.

Changes

• BackfillDagRunResponse / BackfillDagRunCollectionResponse Pydantic models
  with AliasPath for joined DagRun fields (matching existing BackfillResponse pattern)
• list_backfill_dag_runs() route using joinedload(BackfillDagRun.dag_run) — LEFT OUTER JOIN
  includes skipped slots where dag_run_id is NULL
• Pagination via limit/offset, default ordering by sort_ordinal
• 404 when backfill doesn't exist
• 8 unit tests: happy path, skipped slots, 404, pagination, empty backfill, ordering contract

Response shape

{
  "backfill_dag_runs": [
    {
      "id": 1,
      "backfill_id": 1,
      "dag_run_id": 42,
      "logical_date": "2024-01-01T00:00:00Z",
      "partition_key": null,
      "sort_ordinal": 1,
      "exception_reason": null,
      "dag_run_state": "success",
      "dag_run_run_id": "backfill__2024-01-01T00:00:00+00:00"
    },
    {
      "id": 2,
      "backfill_id": 1,
      "dag_run_id": null,
      "logical_date": "2024-01-02T00:00:00Z",
      "partition_key": null,
      "sort_ordinal": 2,
      "exception_reason": "already exists",
      "dag_run_state": null,
      "dag_run_run_id": null
    }
  ],
  "total_entries": 10
}

Follow-up PRs

• UI: Progress bar on backfills table/banner + backfill detail page with Dag runs table
• CLI: airflow backfill list and airflow backfill list-runs commands

closes: #46250

---

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

• Yes — Claude Code (claude-opus-4-6)

Generated-by: Claude Code (claude-opus-4-6) following the guidelines

[shivaam]

@vatsrahul1001 tagging you since you mentioned you were planning to work on it.

[shivaam]

joinedload issues a LEFT OUTER JOIN, preserving rows where dag_run_id is NULL (skipped slots).