refactor(proxy): extract format transform into standalone module

- 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.

src-tauri/src/proxy/providers/adapter.rs

@@ -6,7 +6,6 @@ use super::auth::AuthInfo;
 use crate::provider::Provider;
 use crate::proxy::error::ProxyError;
 use reqwest::RequestBuilder;
-use serde_json::Value;
 
 /// 供应商适配器 Trait
 ///
@@ -83,49 +82,4 @@ pub trait ProviderAdapter: Send + Sync {
     /// # Returns
     /// 添加了认证头的 RequestBuilder
     fn add_auth_headers(&self, request: RequestBuilder, auth: &AuthInfo) -> RequestBuilder;
-
-    /// 是否需要格式转换
-    ///
-    /// 默认返回 `false`（透传模式）。
-    /// 仅当供应商需要格式转换时（如 Claude + OpenRouter 旧 OpenAI 兼容接口）才返回 `true`。
-    ///
-    /// # Arguments
-    /// * `provider` - Provider 配置
-    fn needs_transform(&self, _provider: &Provider) -> bool {
-        false
-    }
-
-    /// 转换请求体
-    ///
-    /// 将请求体从一种格式转换为另一种格式（如 Anthropic → OpenAI）。
-    /// 默认实现直接返回原始请求体（透传）。
-    ///
-    /// # Arguments
-    /// * `body` - 原始请求体
-    /// * `provider` - Provider 配置（用于获取模型映射等）
-    ///
-    /// # Returns
-    /// * `Ok(Value)` - 转换后的请求体
-    /// * `Err(ProxyError)` - 转换失败
-    fn transform_request(&self, body: Value, _provider: &Provider) -> Result<Value, ProxyError> {
-        Ok(body)
-    }
-
-    /// 转换响应体
-    ///
-    /// 将响应体从一种格式转换为另一种格式（如 OpenAI → Anthropic）。
-    /// 默认实现直接返回原始响应体（透传）。
-    ///
-    /// # Arguments
-    /// * `body` - 原始响应体
-    ///
-    /// # Returns
-    /// * `Ok(Value)` - 转换后的响应体
-    /// * `Err(ProxyError)` - 转换失败
-    ///
-    /// Note: 响应转换将在 handler 层集成，目前预留接口
-    #[allow(dead_code)]
-    fn transform_response(&self, body: Value) -> Result<Value, ProxyError> {
-        Ok(body)
-    }
 }