fix: convert OAI ImageUrl to Claude Image format in CC proxy

[DragonFSKY]

Summary

• Fix image conversion in Claude Code proxy's OpenAI-compatible endpoint (/code/v1)
• Convert OpenAI's image_url format to Claude's native image format with source block
• Add normalize_block() function to handle the format transformation
• Includes 10 unit tests covering various image format scenarios

Problem

The /code/v1 endpoint was not properly converting OpenAI's image format to Claude's expected format, causing image content to be ignored.

Changes

• Add normalize_block() function in src/types/oai.rs to convert:
  • image_url with data:image/xxx;base64,... URL → Claude image with source.type = "base64"
  • Preserve text blocks unchanged
  • Filter out unsupported block types

Test Results

Tested with various image sizes (50x50, 100x100, 200x200, 500x500) - all correctly identified colors.

Test plan

• Unit tests pass (cargo test)
• Manual testing with PNG images of various sizes
• Manual testing with JPEG images