## Summary
This replaces the default model provider with a `MultiProvider`, which
has the logic:
- if the model name starts with `openai/` or doesn't contain "/", use
OpenAI
- if the model name starts with `litellm/`, use LiteLLM to use the
appropriate model provider.
It's also extensible, so users can create their own mappings. I also
imagine that if we natively supported Anthropic/Gemini etc, we can add
it to MultiProvider to make it work.
The goal is that it should be really easy to use any model provider.
Today if you pass `model="gpt-4.1"`, it works great. But
`model="claude-sonnet-3.7"` doesn't. If we can make it that easy, it's a
win for devx.
I'm not entirely sure if this is a good idea - is it too magical? Is the
API too reliant on litellm? Comments welcome.
## Test plan
For now, the example. Will add unit tests if we agree its worth mergin.
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Co-authored-by: Steven Heidel <steven@heidel.ca>
