Create AI assistant API session only after the first message

Problem/Motivation

Currently, when the chatbot is added in the site for anonymous users, a new session is started in order to create the thread. This is necessary, but it can be optimized. Creating a session invalidates reverse proxy caches like Varnish or any kind of CDN, decreasing the site's performance. The session may be only created after the first interaction with the chatbot.

Steps to reproduce (required for bugs, but not feature requests)

1. Create an assistant with AI assistant API and AI agents setting up 'Session' in Advanced -> Allow History.
2. Add an ai chatbot block to any page (block layout, layout builder, etc).
3. Enter to the page as anonymous.

After this , a session is started.

Proposed resolution

Only store thread ID after the first message is added by user / system.