librarian: validate library configurations

We noticed that some combinations of configurations are incompatible with a library, e.g., `rest_numeric_enums` is not working with `compute`, which is a DIREGRAPIC library.

Use this issue to track incompatible configurations to a library.

We also need to find a way to return an error early if such configurations exist in `librarian.yaml` or `sdk.yaml`.

Start by locating the validation paths for librarian.yaml and sdk.yaml, then trace how library and configuration combinations are loaded. Done means incompatible configurations such as rest_numeric_enums with compute are detected and reported as an error before generation proceeds.

Written by the indexing model from the issue text.