Update dag endpoint tests to include dags with deadlines

### Description

Need to update tests to create DAGs with deadlines as below:

```
with dag_maker(
DAG1_ID,
dag_display_name=DAG1_DISPLAY_NAME,
schedule=None,
start_date=DAG1_START_DATE,
doc_md="details",
default_args={
"depends_on_past": False,
"retries": 1,
"retry_delay": timedelta(minutes=5),
},
deadline=[DeadlineAlert(
reference=DeadlineReference.DAGRUN_LOGICAL_DATE,
interval=timedelta(hours=1),
callback="dummy.callback",
)],
params={"foo": 1},
tags=["example"],
):
EmptyOperator(task_id=TASK_ID)
```

### Use case/motivation

_No response_

### Related issues

related to https://github.com/apache/airflow/issues/50574

### Are you willing to submit a PR?

- [x] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

Locate the DAG endpoint tests and their existing dag_maker fixtures, then read how the test DAGs are constructed. Update the relevant fixture to include a DeadlineAlert with the shown reference, interval, and callback, and run the targeted endpoint tests to confirm DAGs with deadlines are covered.

Written by the indexing model from the issue text.