Enable JSON mode for Assistant API

JSON mode is essential for building solutions that can consistently and predictably take actions based on Assistant API responses, specifically to deal with results from Runs. Instructing OpenAI to produce JSON in a specified format via prompts is otherwise unpredictable. I’ve spent days improving JSON parsers, until OpenAI APIs respond with yet another variation. JSON mode is already available for Chat API via the response_format parameter. It’s a must have for Assistant APIs too.