model param should be optional for renderPrompt()

[MichaelDoyle]

The prompt config can contain a configured model. We should coalesce the two options. See also #337.

[MichaelDoyle]

I took a closer look at this. I don't actually think model is a supported attribute of a GenerateRequest (which is the return type for a PromptFn). Likely this is a consequence of a typescript limitation, or perhaps we can find a way to tighten this up. Will need to follow up on that.

https://github.com/firebase/genkit/blob/main/js/ai/src/prompt.ts#L28-L30
https://github.com/firebase/genkit/blob/main/js/ai/src/model.ts#L171-L184

It's easy to confuse this with the similarly named GenerateOptions, which is the output of the renderPrompt(...) helper and the input for the generate(...) helper.

cc @AshleyTuring