Ultrafast mode added for GPT-6 Astra in the Responses API

Added Ultrafast mode for GPT-6 Astra in the Responses API. Use gpt-6-astra with service_tier: "ultrafast" to reduce the time between generated output tokens. It is available to API customers, subject to rate limits, with global processing and US data residency. EU and other regional inference residency aren't supported. See Ultrafast pricing.