Scheduler silently skips queued DagRuns forever when their pinned dag version can no longer be resolved

### Apache Airflow version Observed in production on 3.1.8; the code path is unchanged on 3.3.0 and current main (`b628e46`). ### What happened A `DagRun` created by `TriggerDagRunOperator` sat in `QUEUED` for 8+ hours. The scheduler logged, on every loop: ``` DAG 'YB_DUMMY' not found in serialized_dag table ``` while the DAG itself was healthy: not paused, no import errors, visible in the UI, and new runs of the same DAG worked. Run-scoped API endpoints for the stuck run returned 404 while latest-version endpoints returned 200. The parent DAG's `ExternalTaskSensor` treats only `success`/`failed` as terminal, so it poked the queued run until its own 10h timeout. The chain: 1. The run was created **pinned**: `bundle_version` set, `created_dag_version_id` pointing at the dag version current at creation time. The FK is declared `ondelete="set null"` ([dagrun.py L301-303](https://github.com/apache/airflow/blob/main/airflow-core/src/airflow/models/dagrun.py#L301-L303)). 2. The pinned `dag_version` row was later deleted (the FK's `SET NULL` exists precisely because version rows can be deleted). `created_dag_version_id` became NULL **silently**; `bundle_version` stayed set. 3. `DBDagBag._version_from_dag_run` ([dagbag.py L211-216](https://github.com/apache/airflow/blob/main/airflow-core/src/airflow/models/dagbag.py#L211-L216)) only falls back to the latest version when `bundle_version` is **not** set. For a pinned run it returns `created_dag_version_id` — now NULL — so `get_dag_for_run()` returns `None`. 4. `_start_queued_dagruns` handles that with `log.error(...); continue` ([scheduler_job_runner.py, "DAG '%s' not found in serialized_dag table"](https://github.com/apache/airflow/blob/main/airflow-core/src/airflow/jobs/scheduler_job_runner.py)). The run is neither started nor failed, and is re-selected and re-skipped on every subsequent loop, forever. No timeout applies to a QUEUED run in this state. So a single race between "run created & pinned" and "pinned version row d…