Non-existant import

https://github.com/aimclub/ProtoLLM/blob/908c69241bd0231821f6727d2ece9fae211def0c/tests/validation/api_check.py#L3

This module simply does not exist.

I guess it is a typo, and should look like this:

```python
from protollm_tools.llm_api.protollm_api.backend.endpoints import get_router
```