Refactor Image Processing and Error Handling in g4f Client Module

[kqlio67]

This pull request enhances the image processing functionality and error management within the g4f/client/__init__.py file. Key improvements include:

• Robust handling of image data processing in the _process_image_response method
• Explicit conversion handling for image data between URL, base64, and file paths
• Streamlined image processing logic with clearer conditional checks
• Correction of a potential syntax error in the create_variation method
• Enhanced error handling with more specific messages for image data processing

On request: #2363

Additionally, this update includes documentation improvements in docs/async_client.md and docs/client.md.

[hlohaus]

Thank you