Support OpenAI's image_generation tool for multi-turn image editing

[tpaulshippy]

What this does

Builds on top of crmne/ruby_llm#248 and crmne/ruby_llm#325 to enable image generation inside of a chat with OpenAI. See https://platform.openai.com/docs/guides/tools-image-generation for examples and documentation.

Type of change

• New feature

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case

Quality check

• I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

API changes

• No API changes

Related issues

Resolves #2