Allow guest access for AI conversations

Per https://docs.amplify.aws/react/ai/set-up-ai/

`Conversation routes currently ONLY support owner-based authorization and generation routes ONLY support non-owner-based authorization (authenticated, guest, group, publicApiKey).`

I'm trying to make a competitive chat site. Best practices in product growth require low friction adoption. Forcing a signup flow before they understand how good the service is not helping.

Look at chatgpt, you can stay logged out and a get a feel for the service: https://chatgpt.com the same should be possible with amplify ai kit.

Start with the authorization guidance in https://docs.amplify.aws/react/ai/set-up-ai/ and compare the stated rules for conversation and generation routes. Trace how guest or unauthenticated access is represented in the AI conversation flow. Done means a user can try conversations without signing up while the applicable authorization behavior remains covered.

Written by the indexing model from the issue text.