- Create new `proxy/transform/` module with clean architecture:
- `traits.rs`: FormatTransformer trait definition
- `format.rs`: ApiFormat enum (Anthropic, OpenAI, Gemini)
- `registry.rs`: TransformerRegistry with global instance
- `config.rs`: TransformConfig from Provider settings
- `anthropic_openai/`: Bidirectional Anthropic ↔ OpenAI converters
- Move streaming transform from providers/ to transform/
- Remove legacy transform code from providers/adapter.rs and claude.rs
- Update handlers.rs to use new transform module
This refactor improves maintainability and makes it easier to add
new format converters in the future.
YoVinchen