A provider's models may now be provided in the config as a simple
string, or as a mapping with at least a `name` key.
This is a valid configuration:
```yaml
models:
- name: model-a
reasoning: true
- model-b
```
This opens the door to providing model-specific configuration, e.g. an
overridden max tokens count, or whether the model is a 'reasoning'
model.
