### Check for existing issues

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

### What happened?

### What happened

`litellm.validate_environment()` reports the environment as fully configured for zai models even when no `ZAI_API_KEY` is set. Anything that preflights provider credentials with this function (config validation, setup UIs, health checks) gets a green light and then fails later at request time with an auth error

With no `ZAI_API_KEY` in the environment, on current `main` (`3d63eda`):

```python import litellm print(litellm.validate_environment(model="zai/glm-5.1")) ```

returns

``` {'keys_in_environment': True, 'missing_keys': []} ```

Expected, and what the sibling providers correctly return for their key (compare `validate_environment(model="moonshot/kimi-k2")` with no `MOONSHOT_API_KEY`, which reports `missing_keys: ['MOONSHOT_API_KEY']`):

``` {'keys_in_environment': False, 'missing_keys': ['ZAI_API_KEY']} ```

### Root cause

`validate_environment` in `litellm/utils.py` resolves the provider and walks an explicit elif chain that handles 47 providers; `zai` is not one of them. `missing_keys` starts empty and is only popu…