gpt-4.1, gpt-4.1-mini and gpt-4.1-nano added, with up to 1M-token context

Added gpt-4.1, gpt-4.1-mini, and gpt-4.1-nano models to the API. These new models feature improved instruction following, coding, and a larger context window (up to 1M tokens). gpt-4.1 and gpt-4.1-mini are available for supervised fine-tuning. Announced deprecation of gpt-4.5-preview.