Models like mystic, flux-pro-1.1, and seedream-v4/v4-5 require named
aspect ratio slugs (e.g. "square_1_1", "widescreen_16_9") while other
models accept the "W:H" format directly.
- Add normalize_aspect_ratio() mapping W:H strings to slugs for affected models
- Apply normalization in generate-image before building the request payload
- Improve FreepikAPIError to surface invalid_params field details from the API
response, so "Validation error" now also shows which field failed and why
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>