Fix 500 Error in FAB API Users endpoint by allowing naive datetimes i…

[anyeole]

Description

This PR resolves a 500 Internal Server Error that occurs when hitting the FastAPI GET /auth/fab/v1/users endpoint on a live database (e.g., MySQL or Postgres).

The Issue:
The new Pydantic UserResponse datamodel was overly strict, enforcing [UtcDateTime] for the last_login, created_on, and changed_on fields.
Because the FAB provider inherits its SQLAlchemy models directly from Flask-AppBuilder for legacy compatibility, the [ab_user] database schema defines these fields using standard naive sqlalchemy.DateTime.

When SQLAlchemy retrieves these rows from MySQL/Postgres, it returns naive datetime.datetime objects. Pydantic immediately rejects these naive datetimes, crashing the API.

Why this bypassed CI:
The existing unit tests for the /users routes used Python mocks where these timestamp fields were completely omitted (None is permitted by the schema), meaning the strict UtcDateTime validation logic was never tested against standard naive SQLAlchemy database objects.

The Fix:

• Downgraded UtcDateTime to strict datetime.datetime inside the UserResponse schema (providers/fab/src/airflow/providers/fab/auth_manager/api_fastapi/datamodels/users.py) so it properly aligns with the actual SQLAlchemy User model output.
• Updated providers/fab/tests/unit/fab/auth_manager/api_fastapi/datamodels/test_users.py to ensure naive datetimes are correctly preserved rather than strictly coerced or rejected.

Testing/Verification

• Unit tests updated and verified.
• Tested against a live Airflow 3.1.6 environment backed by MySQL (RDS) where the 500 error was successfully resolved.

Related Issues

Fixes: #62325

[boring-cyborg]

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[henry3260]

Thanks for the pr!

[vincbeck]

Looks good, let's see the CI

[henry3260]

Since CI is passing, we don't really need to merge it manually.

[boring-cyborg]

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