`LLMModel` type alias is not callable at runtime

In `bluebox/data_models/llms/vendors.py`, `type LLMModel = OpenAIModel` creates a `TypeAliasType`, which can't be used as a constructor. This means, for example, `LLMModel("gpt-5.1")` raises `TypeError: 'typing.TypeAliasType' object is not callable.`

Change to a plain assignment so it's callable:
```py
# before
type LLMModel = OpenAIModel

# after
LLMModel = OpenAIModel
```