Fix DAG bundle retrieval from S3

[nabuskey]

---

This fixes the case where DAGs stored in S3 are not retrieved. bucket.objects returns prefix objects:

>>> objects =  bucket.objects.filter(Prefix='dags/')
>>> for obj in objects:
...     print(obj)
...
s3.ObjectSummary(bucket_name='bucket', key='dags/')
s3.ObjectSummary(bucket_name='bucket', key='dags/simple.py')

The dags/ object should not be copied as regular files.

Current unit tests do not include the prefix object so the tests passes. However, it does not work when using actual S3 service.

You can reproduce the failure by adding the prefix object in the unit test.

fixes: #57176

Note that the delimiter is hardcoded as /. This is technically not true for all bucket uses. However, since the rest of the code also hardcode delimiter as /, I've left it as-is.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contributors' Guide (https://github.com/apache/airflow/blob/main/contributing-docs/README.rst)
Here are some useful points:

• Pay attention to the quality of your code (ruff, mypy and type annotations). Our prek-hooks will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory). Adding a new operator? Check this short guide Consider adding an example DAG that shows how users should use it.
• Consider using Breeze environment for testing locally, it's a heavy docker but it ships with a working Airflow and a lot of integrations.
• Be patient and persistent. It might take some time to get a review or get the final approval from Committers.
• Please follow ASF Code of Conduct for all communication including (but not limited to) comments on Pull Requests, Mailing list and Slack.
• Be sure to read the Airflow Coding style.
• Always keep your Pull Requests rebased, otherwise your build might fail due to changes not related to your commits.
  Apache Airflow is a community-driven project and together we are making it better 🚀.
  In case of doubts contact the developers at:
  Mailing List: dev@airflow.apache.org
  Slack: https://s.apache.org/airflow-slack

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.