Route SQLAlchemyError through global exception handler

[fat-catTW]

Hello

This change removes the route-local SQLAlchemyError-to-HTTPException handling from the task instance execution route and let the FastAPI app-level exception handlers deal with database errors consistently.

DataError still maps to HTTP 422, while generic SQLAlchemyError now returns HTTP 500 through the shared handler in airflow-core/src/airflow/api_fastapi/common/exceptions.py.
I also added regression coverage to verify both the global handler path and the task_instances route behavior.

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[SameerMesiah97]

I agree with the idea of centralizing the handling of SQLAlchemyError under a single handler i.e. SQLAlchemyErrorHandler, but there are 2 things to keep in mind here:

1. Please check for other places in the API that are catching SQLAlchemyError to make sure they are using the new handler.

2. It appears that you are not just moving the exception handling, but also changing the API response. Previously these endpoints returned {"detail": "Database error occurred"}, whereas they now return the structured payload from SQLAlchemyErrorHandler. I think that's reasonable, but it is an externally observable behaviour change rather than a pure refactor.

Looks good to me otherwise.

[henry3260]

Thanks for contribution :)

[henry3260]

cc @jason810496, what do you think about this?

[jason810496]

LGTM overall, should be ready to merged after addressing the comments from henry and me. Thanks.