feat: model as first parameter in body for proxy routing efficiency

[gbmarc1]

I would like that the generator could produce code with the "model" field as the first parameter only.

Proxies such as open-router (or custom proxies) use the "model" field as a routing parameter. We don't need to read the entire payload for routing, we can readily exit the read when we find the "model" field.

The important thing is that the "model" field should be before the "prompt" or "inputs" as those can be very large.

[RobertCraigie]

Thanks for the PR but I don't think we'll want to change the order of the keys in each method body, instead this ordering could happen in https://github.com/openai/openai-python/blob/main/src/openai/_base_client.py before we turn the dictionary into json, would you mind updating the PR with that approach?