Tests max image count, size, dimensions, and 8k stress test for: - Anthropic, OpenAI, Gemini, Mistral, OpenRouter, xAI, Groq, zAI Key finding: Anthropic's 'many images' rule (>20 images = 2000px max) does NOT cause API errors. 100 x 8k images work fine. Anthropic likely auto-resizes internally. Related to #120