Add ChatbackendInterface

Bumping the target version

I'm not quite getting from the description and a review of the codebase exactly what changes you're suggesting here: is the chatbackendInterface intended as a potential drop in replacement for the AiAssistantRunner that sits on the block forms? As far as I can see the chatbots are just forms wrapped around the ai_assistant_api's functions so it isn't immediately obvious which bit you think might be possible to make more flexible.

Added postponed child issue for when this one is done based on recent discussions:

📌 Separate AI Chatbot documentation from AI Assistant API Postponed