[firestore-genai-chatbot] Allow regenerating response without changing the prompt

### [READ] Step 1: Are you in the right place?

Issues filed here should be about a feature request for a **specific extension in this repository**. To file a feature request that affects multiple extensions or the Firebase Extensions platform, please reach out to
[Firebase support](https://firebase.google.com/support/troubleshooter/report/features/) directly.

### [REQUIRED] Step 2: Extension name

This feature request is for extension: **firestore-genai-chatbot**

### What feature would you like to see?

It would be great to be able to regenerate a response without having to change the prompt.

### How would you use it?

I'm processing the generated response in my code, so it needs to be in a specific format. In some cases the response does not adhere to that format. In these cases I would like to be able to retry with the same prompt.

The current implementation requires me to change the `state` field, as well as changing the prompt in order to regenerate a response.