OpenAIResponseClient.Model is missing

[stephentoub]

A Model property was added to the other client types, e.g. ChatClient.Model, EmbeddingClient.Model, etc. But it appears OpenAIResponseClient was missed.

[jsquire]

@ShivangiReja: Would you please take a look?

[BenjaminDavidPinter]

I've addressed this in this PR, pending approval.

I am aware that this repo doesn't really take PRs, since it's generated from spec. But I figured why not.

[ShivangiReja]

This PR #707 is merged. Thank you @BenjaminDavidPinter for your contribution!