Update models to use SQLA2 annotations and synced model nullable constraints

[vincbeck]

This PR includes 2 changes (hence 2 commits):

• Update all models to use sqlalchemy 2 annotations. The exact same change as Update all models to use sqlalchemy 2 annotations #55954 before it got reverted in Revert "Update all models to use sqlalchemy 2 annotations (#55954)" #56296
• Update models to sync nullable parameter with the annotation. That's the root cause why the CI started to fail after Update all models to use sqlalchemy 2 annotations #55954 got merged.

If you have already reviewed #55954, you need to review only the 2nd commit of this PR.

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[vincbeck]

@pierrejeambrun @Dev-iL, I need your help here. In this PR I update all the models to use sqlalchemy 2 annotations. As part of this migration I also had to fix the different mismtaches between the database and models regarding the non-null constraints. But anyway, long story short is, I updated all the models to use sqlalchemy 2 annotations. All tests are passing but 3, only with python 3.13:

FAILED airflow-core/tests/unit/api_fastapi/core_api/routes/public/test_dag_run.py::TestPatchDagRun::test_patch_dag_run[test_dag1-dag_run_2-patch_body5-response_body5-note_data5] - RuntimeError: No response returned.
FAILED airflow-core/tests/unit/api_fastapi/core_api/routes/public/test_dag_run.py::TestPatchDagRun::test_patch_dag_run_with_update_mask[query_params4-patch_body4-response_body4-200-note_data4] - RuntimeError: No response returned.
FAILED airflow-core/tests/unit/api_fastapi/core_api/routes/public/test_task_instances.py::TestPatchTaskInstance::test_update_mask_set_note_should_respond_200[None-ti_note_data1] - RuntimeError: No response returned.

See the full stack trace here.

I do not quite understand the root cause. Surprisingly it fails only with patch APIs. If you have a bit of time, I'd appreciate if you can take a look and maybe you'll find out something I did not.

[potiuk]

I also added "all versions" to make sure that all python versions are tested. Closed/reopened to trigger it

[vincbeck]

Green 🟢