Change `model_validator(mode="after")` to `model_validator(mode="before")`

[lord-haffi]

This is useful for projects using this library especially for Unittests if you are using model_construct a lot. This is actually due to a bug (or unintendend side effect), see here pydantic/pydantic#6978.

For instance, this helps to resolve issues in PR https://github.com/Hochfrequenz/powercloud2lynqtech/pull/1816.

Otherwise, you would have to use model_construct on every nested structure.