### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

get_model_group_info() crashes with KeyError: 'hidden' when a model_group_alias entry is configured as a dict without the 'hidden' key. Since TypedDict is type-hint only, Python does not inject missing keys at runtime. This crashes every single request because get_model_group_info() is called on every router completion via set_response_headers.

Expected behavior: missing 'hidden' key should be treated as hidden=False.

I have a fix ready and will submit a PR immediately after filing this issue.

### Steps to Reproduce

1. Install litellm: pip install litellm

2. Run the following Python script:

from litellm import Router router = Router( model_list=[{"model_name": "gpt-4", "litellm_params": {"model": "openai/gpt-4", "api_key": "sk-fake"}}], model_group_alias={"my-alias": {"model": "gpt-4"}} ) router.get_model_group_info("my-alias")

3. Observe KeyError: 'hidden' is raised

### Relevant log output

```shell KeyError: 'hidden' File "litellm/router.py", line 8700, in get_model_group_info if item["hidden"] is True: ```

### What par…