Fix CLI dag parsing issues during reserialization

[kunaljubce]

Fixes #59003.

Issue 1: airflow dags reserialize fails to import related modules in CLI while working fine on UI: ModuleNotFoundError: No module named 'module.task_b'

Fix: So this was happening because when airflow tries to load the new module in:

airflow/airflow-core/src/airflow/dag_processing/dagbag.py

Line 442 in 1585e45

loader.exec_module(new_module)

The airflow cli that runs airflow dags reserialize loads each DAG file with a synthetic, top‑level module name via SourceFileLoader and immediately executes it, but it never adds the DAG’s directory, i.e. /files/dags in this case, to sys.path. Once we add the helper package to the sys.path, it works as expected.

Issue 2: airflow tasks clear my_dag complains: AttributeError: 'DAG' object has no attribute 'clear'

Fix: This was happening because of a discrepancy in the type of the dag object returned by the task_clear() function in:

airflow/airflow-core/src/airflow/cli/commands/task_command.py

Lines 472 to 477 in cf80ae1

	if args.dag_id and not args.bundle_name and not args.dag_regex and not args.task_regex:
	dags = [get_dag_by_file_location(args.dag_id)]
	else:
	# todo clear command only accepts a single dag_id. no reason for get_dags with 's' except regex?
	# Reading from_db because clear method still not implemented in Task SDK DAG
	dags = get_dags(args.bundle_name, args.dag_id, use_regex=args.dag_regex, from_db=True)

Specifically, in the if block, the call to get_dag_by_file_location() was returning a DAG object which has no clear() attribute, whereas in the else block, the call to get_dags() returns a SerializedDAG object which the expected clear() attribute. Since this get_dag_by_file_location() does not seem to be called elsewhere, I simply altered the way we retrieve dags here from the DagModel to the SerializedDagModel.

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[kunaljubce]

Hello Maintainers - This is my first PR modifying a core component of dag processing, as far as I can tell. Please feel free to suggest if I am overlooking some other point of possible failure. Thanks!

[Nataneljpwd]

Added a few comments, maybe it is worth trying to change the loader

[Nataneljpwd]

It looks like a patch only of it is 1 directory above or below, what happens when a task is nested 2 modules deep?

[Nataneljpwd]

Do we need this intermediate variable? As it will be in position 0 only when we add it there

[kunaljubce]

@Nataneljpwd You're right, this is an overkill. I will remove it.

[Nataneljpwd]

Maybe we should change to not use a source file loader and instead use spec_from_file_location and use the loader withing the spec?
I think it can solve this issue

[kunaljubce]

Added a few comments, maybe it is worth trying to change the loader

Thanks, I will take a look and make the changes over the next few days :)