fix: use fresh session in FabAuthManager.deserialize_user (#62903)

[davidfrickert]

FabAuthManager.deserialize_user was using self.session (FAB's appbuilder-scoped SQLAlchemy session). When that session becomes poisoned after a DB connection drop, the next request on the same worker raises an error due to attempting to use dropped connection. This can cause a Internal Server Error to the user.

Replace self.session with create_session() so each cache miss gets a fresh, short-lived session that is closed immediately after the query. Also replace manual query with get function which returns an optional value instead of exception when not found.

---

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Used Claude code. Reviewed code and tested.

Fixes #62903

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst or {issue_number}.significant.rst, in airflow-core/newsfragments.

[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

[vincbeck]

#62919 is already working on fixing the issue. Closing