Initial prompts being dropped

[mikechao]

This PR fixes the issue where the initialPrompts property are being dropped when creating a browserAI.

For example

browserAI("text", {
      expectedInputs: [{ type: "text" }, { type: "image" }, { type: "audio" }],
      initialPrompts: [{ role: "system", content: "You will talk like a pirate" }],
      onContextOverflow: this.onContextOverflow,
    });

The system prompt of "You will talk like a pirate" is discarded.

Below is the example chat app where I added a system prompt panel. And in the new panel I put "You will talk like a pirate and use a lot of pirate related emojis"

[vercel]

@mikechao is attempting to deploy a commit to the browser-ai OSS Program Team on Vercel.

A member of the Team first needs to authorize it.