Scheduler exception handling in dagrun create loop doesn't work

### Body

In SchedulerJobRunner, in _create_dag_runs, we do `except Exception: ... continue` but this does not work when the exception is the result of a db error; in that case you need to roll the transaction back.

to fix this we need either smaller transactions (e.g. one per dag run create attempt) or savepoints. either way it's a bit of a refactor.

### Committer

- [x] I acknowledge that I am a maintainer/committer of the Apache Airflow project.

Start in SchedulerJobRunner._create_dag_runs and inspect how the database transaction spans each DAG run creation attempt. Compare using smaller transactions or savepoints, then verify that a database exception rolls back its transaction and the loop can continue with later DAG runs.

Written by the indexing model from the issue text.