Walk each Dag once in DBDagBag.iter_all_latest_version_dags

[Eason09053360]

DBDagBag.iter_all_latest_version_dags() promises the latest version, but the
query returned every serialized_dag row, so a Dag was walked once per version
it had. Its one production caller is the FAB auth manager's Dag permission sync,
which therefore re-synced the same dag_id once per version.

Same origin and same fix as #70934: reuse the select that picks the highest
version_number per dag_id. That select is now used from a second module, so it
loses its leading underscore.

SerializedDagModel.read_all_dags() is not used here on purpose — it materialises
every Dag into a dict and ignores load_op_links, while this method is
deliberately a non-caching generator.

related: #70934

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• Yes — Claude Code (Opus 5)

Generated-by: Claude Code (Opus 5) following the guidelines

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[Eason09053360]

Closing as a duplicate of #70935, which fixes the same bug