generated code is not decorated with GeneratedCodeAttribute

The code generated by the grains code generator is not decorated with the `GeneratedCodeAttribute`, which results in unnecessary build messages, as can be seen on the screenshot, which would normally not happen as analyzers skip generated code:


<img width="1164" height="1588" alt="Image" src="https://github.com/user-attachments/assets/e0ea511d-fd4e-4ca1-9ad3-7a1f71c9f53e" />