Fix XCom slice endpoint returning wrong rows for crossed-bound slices

[dhruv-techdev]

Fix the mapped XCom sequence-slice API returning incorrect rows for crossed-bound slices, where stop comes before start.

In Python, crossed-bound slices such as items[5:2] return an empty list. However, the mapped-XCom slice endpoint translated this case into a SQLAlchemy .slice() call that could produce a negative SQL LIMIT.

This caused inconsistent behavior across databases:

SQLite treated the negative LIMIT as “no limit”, so it silently returned incorrect rows from the offset onward.

PostgreSQL/MySQL rejected the negative LIMIT, causing the request to fail.

This PR adds explicit handling for crossed-bound slices so the endpoint returns an empty result consistently across supported database backends, matching Python slicing behavior.

Tests were added/updated to cover the crossed-bound slice case.

closes: #69204

[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
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