Upgrade chat completions model to gpt-6-astra and add steering support

This update replaces the chat completions model from gpt-5.6-sol to gpt-6-astra in all examples and responses, aligning with the latest model capabilities. It introduces new websocket events and API schema components to enable response steering, allowing clients to queue user input to influence ongoing responses, with events for accepted, pending, and failed steering inputs. Additional enhancements include support for asynchronous tool responses and improved error and status handling for chat threads.

OpenAI-API-Ref-Source-Revision: 1052570