Fix: allow deadline alert UUID references in serialized Dag schema

[sjyangkevin]

• related: Move deadline_alerts into their own table for UI integration #58248
• related: Feat/kubernetes executor callback support #67449

When a Dag defines a deadline alert, the alert is stored in its own deadline_alert table on write, and SerializedDagModel._generate_deadline_uuids rewrites the serialized Dag's deadline field from a list of encoded dicts into a list of UUID strings pointing at those rows. However, schema.json only allowed deadline to be a dict, an array of dicts, or null. As a result, validating the stored serialized Dag for any deadline-bearing Dag fails with something like:

FAILED airflow-core/tests/unit/models/test_serialized_dag.py::TestSerializedDagModel::test_write_dag[compress-serialized_dags] - jsonschema.exceptions.ValidationError: ['019ea3de-70d1-77ec-82b1-0dceeb508be5'] is not valid under any of the given schemas

Failed validating 'anyOf' in schema['allOf'][0]['properties']['dag']['properties']['deadline']:
    {'anyOf': [{'$ref': '#/definitions/dict'},
               {'items': {'$ref': '#/definitions/dict'}, 'type': 'array'},
               {'type': 'null'}]}

On instance['dag']['deadline']:
    ['019ea3de-70d1-77ec-82b1-0dceeb508be5']

Fix

• Adds an array of string option to the deadline schema so it accepts the persisted UUID-reference format.
• Adds a regression test that writes a deadline-bearing Dag and asserts the stored data passes schema validation, guarding against the schema and storage format drifting apart again.

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