Responses API adds async tool calling and mid-turn steering for GPT-6 Astra

Added new controls for long-running work with GPT-6 Astra in the Responses API:

• Async tool calling: Let the model continue working while your application runs function or custom tools, then return results as they become available.
• Mid-turn steering: Send additional instructions while a response is in progress over WebSockets, so the model can incorporate corrections or changing requirements.
• Change reasoning effort mid-conversation: Increase effort for difficult work or reduce it for routine follow-ups while preserving the cached prompt prefix.