Add gpt-image-2 models to image generation and editing APIs

This update introduces two new GPT image generation models, `gpt-image-2` and `gpt-image-2-2026-04-21`, expanding the available model options across image editing and generation endpoints. The documentation and model enums have been updated to reflect support for these models along with existing ones, enhancing image input capabilities and model selection consistency.

OpenAI-API-Ref-Source-Revision: 989265