Flush scheduled DagRun creation state promptly

[fat-catTW]

Why

SchedulerJobRunner._create_dag_runs() had a TODO asking whether the scheduler should call session.flush() or session.expunge_all() while creating scheduled DagRuns.

The concern behind the TODO is that the scheduler may iterate over multiple Dags, create scheduled DagRuns, update DagModel scheduling fields, and create task instance rows in the same SQLAlchemy session. For larger Dags or larger batches of Dags, leaving ORM state pending until the outer transaction boundary can make it harder to reason about session growth and memory behavior.

This PR resolves that TODO by flushing after each successful scheduled DagRun creation and related DagModel scheduling-state update.

Why flush() Instead of expunge_all()

The narrower operation is enough for the state observed in this path.

session.flush() writes pending ORM changes to the database transaction while keeping objects attached to the session. That directly addresses the observed pending/dirty scheduler state after creating a scheduled DagRun and updating the corresponding DagModel.

session.expunge_all() does something broader: it detaches every ORM object from the session. That can reduce identity-map retention, but it also carries more behavioral risk because later scheduler code may still expect ORM objects to be attached, refreshable, or tracked by the session.

To decide whether expunge_all() was needed, this path was checked for retained ORM objects after DagRun creation. If creating larger Dags left one tracked DagRun or TaskInstance object per created row, flush() would not address that memory growth because it does not clear the session identity map.

A local pressure check with 20 Dags and 500 tasks per Dag created 10,000 TaskInstance rows. After _create_dag_runs() returned, the observed session state was:

session.new: 0
session.dirty: 0
identity_map: Counter({'DagModel': 20, 'DagVersion': 1, 'SerializedDagModel': 1, 'DagCode': 1})

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