Fix deadline pruning crash when dag.deadline holds serialized alerts

[wislertt]

Why

airflow dags test exits non-zero for any DAG that sets a deadline, even though the run succeeded. DAG.test() builds its scheduler DAG with an in-memory serialize/deserialize round trip, which leaves dag.deadline holding encoded deadline alerts instead of deadline_alert ids. When the run succeeds, DagRun.update_state() passes those dicts to DeadlineAlertModel.get_by_id(), which fails in the Uuid bind processor with 'dict' object has no attribute 'hex'.

What

Dict entries in dag.deadline are decoded directly instead of looked up in the deadline_alert table. Entries that are strings keep the existing get_by_id() path used by the scheduler.

closes: #73428

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