Determine latest Dag version by version_number, not created_at

[ephraimbuddy]

The latest DagVersion was selected by created_at DESC, which
is not deterministic when two versions share a timestamp (or under clock skew).
write_dag derives the next version_number from that row, so picking a non-max row collided with the (dag_id, version_number) unique constraint. Order by the monotonic, unique version_number instead, consistently across get_latest_version/write_dag, get_version, and the bulk prefetch.

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  Claude opus 4.8

[Copilot]

Pull request overview

This PR fixes nondeterministic “latest DagVersion” selection by ordering on version_number (monotonic/unique per dag_id) instead of created_at, preventing occasional collisions with the (dag_id, version_number) unique constraint when timestamps tie or clock skew occurs.

Changes:

• Update DagVersion “latest” selection logic to order by version_number (including get_latest_version, get_version, and the internal select used by write_dag).
• Update bulk prefetch logic in SerializedDagModel._prefetch_dag_write_metadata to align “latest” selection with DagVersion’s version_number.
• Add regression tests that seed DagVersions where created_at ordering disagrees with version_number ordering.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
airflow-core/src/airflow/models/dag_version.py	Switches “latest” DagVersion selection to deterministic version_number DESC ordering.
airflow-core/src/airflow/models/serialized_dag.py	Aligns bulk prefetch window ordering with DagVersion selection by joining and ordering on DagVersion.version_number.
airflow-core/tests/unit/models/test_dag_version.py	Adds regression coverage for inverted created_at vs version_number and verifies write_dag increments from max version.