Bring back AI models as `const`

In commit 7f3789cac41ea0f91b1de148cc80c938a51f1c3c, `modelMappings` was removed and replaced by creating union types directly in the argument in the `run()` overloads. This removes the ability to get the models available at runtime.

Also putting the model names directly inline to each overload makes it difficult to retrieve the models for a task type outside of the `run()` method vs. externalizing each one individually (or as part of a namespace or other grouping method).