Add retryability strategies

[gerardatkonvo]

Hi! Really excited about this project.

Currently one of the biggest pain points with Vercel's ai SDK is that there is no built-in way for configuring custom retry logic. We need to write a custom adapter to handle this like https://github.com/remorses/ai-fallback or use some gateway/router that does this.

It his a highly requested feature (vercel/ai#2636)

How the API could look like:

    const adapterModelList = [{ model: 'gpt-4o', adapter: openai()}, { model: 'claude-sonnet-4-latest', adapter: anthropic() }]
    const stream = chat({
      getAdapterModel(previousError, iteration) {
        if (!previousError || (previousError.httpCode >= 500 && iteration < adapterModelList.length)) {
          return adapterModelList[iteration]
        }
        throw new Error(`Can't retry`, { cause: previousError })
      }
      messages,
      conversationId
    });