feat(ai): add image generation domain #37713

Summary

Add image generation as a separate semantic domain in @opencode-ai/ai, while keeping conversational image generation in the existing LLM interaction model.

Initial scope

• Add provider-neutral ImageRequest, ImageModel, ImageResponse, and Image.generate(...) APIs.
• Implement direct OpenAI Images generation through POST /v1/images/generations.
• Expose OpenAI's hosted image_generation tool through the Responses route for conversational generation and editing.
• Normalize generated image output while preserving provider metadata and usage.

Protocol direction

There is no ecosystem-wide image wire protocol comparable to Chat Completions. OpenAI, xAI, Together, and DeepInfra share a useful /images/generations compatibility subset, but Google, Stability, BFL, Replicate, fal, and Fireworks use materially different multimodal, multipart, or asynchronous job protocols.

Treat OpenAI Images as a reusable provider-family adapter, not as the package's universal image model. Keep the public request semantic and add provider adapters underneath it.