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/adapter.go

@@ -0,0 +1,100 @@
+package conversion
+
+import (
+	"fmt"
+	"sync"
+
+	"nex/backend/internal/conversion/canonical"
+)
+
+// ProtocolAdapter 协议适配器接口
+type ProtocolAdapter interface {
+	ProtocolName() string
+	ProtocolVersion() string
+	SupportsPassthrough() bool
+
+	DetectInterfaceType(nativePath string) InterfaceType
+	BuildUrl(nativePath string, interfaceType InterfaceType) string
+	BuildHeaders(provider *TargetProvider) map[string]string
+	SupportsInterface(interfaceType InterfaceType) bool
+
+	DecodeRequest(raw []byte) (*canonical.CanonicalRequest, error)
+	EncodeRequest(req *canonical.CanonicalRequest, provider *TargetProvider) ([]byte, error)
+	DecodeResponse(raw []byte) (*canonical.CanonicalResponse, error)
+	EncodeResponse(resp *canonical.CanonicalResponse) ([]byte, error)
+
+	CreateStreamDecoder() StreamDecoder
+	CreateStreamEncoder() StreamEncoder
+
+	EncodeError(err *ConversionError) ([]byte, int)
+
+	DecodeModelsResponse(raw []byte) (*canonical.CanonicalModelList, error)
+	EncodeModelsResponse(list *canonical.CanonicalModelList) ([]byte, error)
+	DecodeModelInfoResponse(raw []byte) (*canonical.CanonicalModelInfo, error)
+	EncodeModelInfoResponse(info *canonical.CanonicalModelInfo) ([]byte, error)
+	DecodeEmbeddingRequest(raw []byte) (*canonical.CanonicalEmbeddingRequest, error)
+	EncodeEmbeddingRequest(req *canonical.CanonicalEmbeddingRequest, provider *TargetProvider) ([]byte, error)
+	DecodeEmbeddingResponse(raw []byte) (*canonical.CanonicalEmbeddingResponse, error)
+	EncodeEmbeddingResponse(resp *canonical.CanonicalEmbeddingResponse) ([]byte, error)
+	DecodeRerankRequest(raw []byte) (*canonical.CanonicalRerankRequest, error)
+	EncodeRerankRequest(req *canonical.CanonicalRerankRequest, provider *TargetProvider) ([]byte, error)
+	DecodeRerankResponse(raw []byte) (*canonical.CanonicalRerankResponse, error)
+	EncodeRerankResponse(resp *canonical.CanonicalRerankResponse) ([]byte, error)
+}
+
+// AdapterRegistry 适配器注册表接口
+type AdapterRegistry interface {
+	Register(adapter ProtocolAdapter) error
+	Get(protocolName string) (ProtocolAdapter, error)
+	ListProtocols() []string
+}
+
+// memoryRegistry 基于内存的适配器注册表
+type memoryRegistry struct {
+	mu       sync.RWMutex
+	adapters map[string]ProtocolAdapter
+}
+
+// NewMemoryRegistry 创建内存注册表
+func NewMemoryRegistry() AdapterRegistry {
+	return &memoryRegistry{
+		adapters: make(map[string]ProtocolAdapter),
+	}
+}
+
+// Register 注册适配器
+func (r *memoryRegistry) Register(adapter ProtocolAdapter) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	name := adapter.ProtocolName()
+	if _, exists := r.adapters[name]; exists {
+		return fmt.Errorf("适配器已注册: %s", name)
+	}
+	r.adapters[name] = adapter
+	return nil
+}
+
+// Get 获取适配器
+func (r *memoryRegistry) Get(protocolName string) (ProtocolAdapter, error) {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+
+	adapter, ok := r.adapters[protocolName]
+	if !ok {
+		return nil, fmt.Errorf("未找到适配器: %s", protocolName)
+	}
+	return adapter, nil
+}
+
+// ListProtocols 列出所有已注册协议
+func (r *memoryRegistry) ListProtocols() []string {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+
+	protocols := make([]string, 0, len(r.adapters))
+	for name := range r.adapters {
+		protocols = append(protocols, name)
+	}
+	return protocols
+}