refactor: 实现 ConversionEngine 协议转换引擎，替代旧 protocol 包

引入 Canonical Model 和 ProtocolAdapter 架构，支持 OpenAI/Anthropic 协议间
无缝转换，统一 ProxyHandler 替代分散的 OpenAI/Anthropic Handler，简化
ProviderClient 为协议无关的 HTTP 发送器，Provider 新增 protocol 字段。

backend/internal/conversion/middleware.go

@@ -0,0 +1,76 @@
+package conversion
+
+import (
+	"time"
+
+	"nex/backend/internal/conversion/canonical"
+
+	"github.com/google/uuid"
+)
+
+// ConversionMiddleware 转换中间件接口
+type ConversionMiddleware interface {
+	Intercept(req *canonical.CanonicalRequest, clientProtocol, providerProtocol string, ctx *ConversionContext) (*canonical.CanonicalRequest, error)
+	InterceptStreamEvent(event *canonical.CanonicalStreamEvent, clientProtocol, providerProtocol string, ctx *ConversionContext) (*canonical.CanonicalStreamEvent, error)
+}
+
+// ConversionContext 转换上下文
+type ConversionContext struct {
+	ConversionID  string
+	InterfaceType InterfaceType
+	Timestamp     time.Time
+	Metadata      map[string]any
+}
+
+// NewConversionContext 创建转换上下文
+func NewConversionContext(ifaceType InterfaceType) *ConversionContext {
+	return &ConversionContext{
+		ConversionID:  uuid.New().String(),
+		InterfaceType: ifaceType,
+		Timestamp:     time.Now().UTC(),
+		Metadata:      make(map[string]any),
+	}
+}
+
+// MiddlewareChain 中间件链
+type MiddlewareChain struct {
+	middlewares []ConversionMiddleware
+}
+
+// NewMiddlewareChain 创建中间件链
+func NewMiddlewareChain() *MiddlewareChain {
+	return &MiddlewareChain{
+		middlewares: make([]ConversionMiddleware, 0),
+	}
+}
+
+// Use 添加中间件
+func (c *MiddlewareChain) Use(mw ConversionMiddleware) {
+	c.middlewares = append(c.middlewares, mw)
+}
+
+// Apply 对请求按顺序执行所有中间件
+func (c *MiddlewareChain) Apply(req *canonical.CanonicalRequest, clientProtocol, providerProtocol string, ctx *ConversionContext) (*canonical.CanonicalRequest, error) {
+	result := req
+	for _, mw := range c.middlewares {
+		var err error
+		result, err = mw.Intercept(result, clientProtocol, providerProtocol, ctx)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return result, nil
+}
+
+// ApplyStreamEvent 对流式事件按顺序执行所有中间件
+func (c *MiddlewareChain) ApplyStreamEvent(event *canonical.CanonicalStreamEvent, clientProtocol, providerProtocol string, ctx *ConversionContext) (*canonical.CanonicalStreamEvent, error) {
+	result := event
+	for _, mw := range c.middlewares {
+		var err error
+		result, err = mw.InterceptStreamEvent(result, clientProtocol, providerProtocol, ctx)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return result, nil
+}