Incompatibility with Pydantic 2.11

Using pydantic 2.11 will break the tests. 

At the moment to me the exact reason / the nature of the error is unclear to me. 
However, since many models fail, it might be related to something in the PyRadPlanBaseModel.

For now, I have restricted pydantic to <2.11 in the pyproject.toml.

If someone wants to look into this, please let me know.

