o3-deep-research and o4-mini-deep-research released, plus webhooks and cheaper web search

Released o3-deep-research and o4-mini-deep-research, deep research variants of our o-series reasoning models optimized for deep analysis and research tasks. Learn more in the deep research guide.

Added support for async event handling with webhooks. Reduced and simplified pricing for the web search tool. Added support for the web search tool.