Add deadline alert coverage to DAG endpoint tests

[shivaam]

Summary

• Add tests verifying all public DAG API endpoints work correctly with deadline-bearing DAGs
• Cover GET /dags, GET /dags/{id}, GET /dags/{id}/details, PATCH /dags/{id}, PATCH /dags, and DELETE /dags/{id}
• Add clear_db_deadline and clear_db_deadline_alert to test cleanup to prevent FK constraint issues

Details

The public DAG endpoint tests (test_dags.py) never created DAGs with deadlines, so there was zero coverage for how the API behaves when DagModel._deadline has data and SerializedDagModel.deadline_alerts has FK-linked records.

7 new test methods across 6 test classes. No existing tests modified.

Note: DAGResponse and DAGDetailsResponse do not yet include deadline fields (that work is tracked separately in #50574), so these tests verify the endpoints return correct responses without errors — not the deadline data itself. Once response fields are added, these tests can be extended with deadline-specific assertions.

closes: #50992

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Opus 4.6)

Generated-by: Claude Code (Opus 4.6) following the guidelines

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