fix(openrouter): Handle null id/model in API responses.

[pocketarc]

OpenRouter can sometimes return responses without id or model fields in the response body. The handlers passed these values directly to Meta::__construct() without defaults, causing a TypeError when they were null.

Before

TypeError: Prism\Prism\ValueObjects\Meta::__construct(): Argument #1 ($id) must be of type string, null given

After

The response is handled, with id defaulting to an empty string and model falling back to the requested model from the original request.