Make backfill creation robust against scheduler race condition

### Description The `_create_backfill()` method in `airflow-core/src/airflow/models/backfill.py` creates a backfill in two steps: 1. It commits the `Backfill` row to the database (`session.commit()`) 2. Then it creates all the `DagRun` and `BackfillDagRun` rows linked to that backfill The scheduler runs `_mark_backfills_complete()` every ~30 seconds, looking for backfills where all associated DagRuns have finished. If the scheduler runs between Step 1 and Step 2, it sees a backfill with zero DagRuns, concludes "all zero runs are done", and marks the backfill as complete. When Step 2 then creates the DagRuns, they're orphaned. PR #62561 fixed this with a pragmatic guard: require at least one `BackfillDagRun` row before marking complete, with a 2-minute timeout to clean up orphaned backfills that failed during initialization. This works, but relies on a time-based heuristic rather than addressing the root cause. During [PR #62561 review](https://github.com/apache/airflow/pull/62561#pullrequestreview-4030269060), Daniel Standish suggested two more robust approaches as follow-ups: **Approach 1: Atomic transaction** — Wrap the entire backfill creation in a single transaction so `Backfill`, `DagRun`, and `BackfillDagRun` rows all appear atomically. One way: change `session.commit()` to `session.flush()` in `_create_backfill()` ([backfill.py L605](https://github.com/apache/airflow/blob/main/airflow-core/src/airflow/models/backfill.py#L605)). `flush()` assigns the ID without committing; `create_session()` commits everything at the end. Tradeoff: the [`AlreadyRunningBackfill` check](https://github.com/apache/airflow/blob/main/airflow-core/src/airflow/models/backfill.py#L585-L589) relies on the early commit to block concurrent duplicates, so this would need database-level locking. (Also suggested by [Kaxil](https://github.com/apache/airflow/pull/62561#discussion_r2880363898)) **Approach 2: Add a state field or datetime** — Add an explicit signal to distinguish "initializing" …