Fix flakey test in DAG serialization (apache#44480)

This behaviour is very odd, and is not in anyway clear _why_ it's happening,
but the fix is to not construct a DAG object, but to use attrs to tell us what
the fields are -- which is better behaviour anyway.

To reproduce the issue before this change:

1. `pytest tests/serialization/test_dag_serialization.py::TestStringifiedDAGs::test_deserialization_with_dag_context` -> greeen
2. `pytest tests/serialization/test_dag_serialization.py::TestStringifiedDAGs::test_deserialization_with_dag_context` -> boom

(So by dag objects existing in the DB it somehow "poluted" the
DagContextManager/TaskContextManager stack. I haven't dug into exactly how
that might be happening.)

task_sdk/src/airflow/sdk/definitions/dag.py

@@ -991,7 +991,7 @@ def get_serialized_fields(cls):
                 "fail_stop",
                 "schedule",
             }
-            cls.__serialized_fields = frozenset(vars(DAG(dag_id="test", schedule=None))) - exclusion_list
+            cls.__serialized_fields = frozenset(a.name for a in attrs.fields(cls)) - exclusion_list
         return cls.__serialized_fields
 
     def get_edge_info(self, upstream_task_id: str, downstream_task_id: str) -> EdgeInfoType: