Deduplicate error docs, fix image sizes, add Mistral vision example

[mintlify]

Summary

• Deduplicate error handling content: Reduced concepts/error-handling.mdx from ~540 lines to ~190 lines by removing duplicated class definitions and constructor signatures already covered in api/core/errors.mdx. The concept page now serves as a concise usage guide with a clear link to the API reference for full details.
• Fix incorrect image sizes: Removed references to 256x256 and 512x512 sizes in the image generation guide, which are legacy DALL-E sizes not applicable to gpt-image-1. Updated all code examples to use valid sizes (1024x1024, 1792x1024, 1024x1792).
• Add Mistral vision example: The Mistral provider capabilities table lists "Vision: Yes" but had no corresponding code example. Added a vision/multimodal example demonstrating image input.