Custom model names can include the `@` symbol by itself.

To specify the model's provider, append it after the model name using `@` as before.

This format supports cases like `google vertex ai` with a model name like `claude-3-5-sonnet@20240620`.

For instance, `claude-3-5-sonnet@20240620@vertex-ai` will be split by `split(/@(?!.*@)/)` into:

`[ 'claude-3-5-sonnet@20240620', 'vertex-ai' ]`, where the former is the model name and the latter is the custom provider.