Provider registry enables use of other providers

[martinemde]

This refactoring makes it possible to support more providers than the current two (openrouter and openai) without modifying the existing interface, fallback behavior, or expectations for consumers.

[martinemde]

This was inspired by the need for raix-bedrock, which I just published. It can work without this, but I think the API is cleaner with this change.

[martinemde]

One reason not to merge this is that it doesn't go far enough.

The interface inside of raix-bedrock is not pretty. I have to do a lot of work to change formats to use the AWS bedrock gem, then a lot more work to change back.

It would be great if we moved enough of the complexity so that the interface was more like "register tool, tool called, chat completion, completion result" in an abstract way so that the OpenAI response structure and the bedrock structure was hidden into the OpenAI provider and bedrock provider. Right now, I only normalized the call sites but none of the params or return formats.

I'll take another stab at this when I have a moment. Let me know if you don't think it's worth the effort or you wouldn't want to merge it.