Fix paint not working with OpenRouter provider (#513)

[khasinski]

OpenRouter uses the chat completions endpoint for image generation instead of a dedicated /images/generations endpoint. This commit adds an OpenRouter::Images module that:

• Routes image generation requests to chat/completions
• Formats the payload with modalities: ["image", "text"]
• Parses the response from choices[0].message.images[]
• Handles both base64 data URLs and regular URLs

What this does

When using RubyLLM.paint with OpenRouter, requests were being sent to https://openrouter.ai/api/v1/images/generations, which doesn't exist. OpenRouter returns a 405 Method Not Allowed error.

OpenRouter implements image generation via the chat completions endpoint (/chat/completions) with modalities: ["image", "text"], not through a dedicated images endpoint.

This PR adds an OpenRouter::Images module that overrides the inherited OpenAI::Images behavior to:

1. Route to the correct endpoint: chat/completions instead of images/generations
2. Format the request correctly: Uses chat message format with modalities: ["image", "text"]
3. Parse the response correctly: Extracts images from choices[0].message.images[] as base64 data URLs

Tested with google/gemini-2.5-flash-image-preview via OpenRouter.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

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
  • For provider changes: Re-recorded VCR cassettes with bundle exec rake vcr:record[provider_name]
  • All tests pass: bundle exec rspec
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

API changes

• Breaking change
• New public methods/classes
• Changed method signatures
• No API changes

Related issues

Fixes #513