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/openai/adapter.go

@@ -0,0 +1,211 @@
+package openai
+
+import (
+	"encoding/json"
+	"regexp"
+
+	"nex/backend/internal/conversion"
+	"nex/backend/internal/conversion/canonical"
+)
+
+// Adapter OpenAI 协议适配器
+type Adapter struct{}
+
+// NewAdapter 创建 OpenAI 适配器
+func NewAdapter() *Adapter {
+	return &Adapter{}
+}
+
+var modelInfoRegex = regexp.MustCompile(`^/v1/models/[^/]+$`)
+
+// ProtocolName 返回协议名称
+func (a *Adapter) ProtocolName() string { return "openai" }
+
+// ProtocolVersion 返回协议版本
+func (a *Adapter) ProtocolVersion() string { return "" }
+
+// SupportsPassthrough 支持同协议透传
+func (a *Adapter) SupportsPassthrough() bool { return true }
+
+// DetectInterfaceType 根据路径检测接口类型
+func (a *Adapter) DetectInterfaceType(nativePath string) conversion.InterfaceType {
+	switch {
+	case nativePath == "/v1/chat/completions":
+		return conversion.InterfaceTypeChat
+	case nativePath == "/v1/models":
+		return conversion.InterfaceTypeModels
+	case modelInfoRegex.MatchString(nativePath):
+		return conversion.InterfaceTypeModelInfo
+	case nativePath == "/v1/embeddings":
+		return conversion.InterfaceTypeEmbeddings
+	case nativePath == "/v1/rerank":
+		return conversion.InterfaceTypeRerank
+	default:
+		return conversion.InterfaceTypePassthrough
+	}
+}
+
+// BuildUrl 根据接口类型构建 URL
+func (a *Adapter) BuildUrl(nativePath string, interfaceType conversion.InterfaceType) string {
+	switch interfaceType {
+	case conversion.InterfaceTypeChat:
+		return "/v1/chat/completions"
+	case conversion.InterfaceTypeModels:
+		return "/v1/models"
+	case conversion.InterfaceTypeEmbeddings:
+		return "/v1/embeddings"
+	case conversion.InterfaceTypeRerank:
+		return "/v1/rerank"
+	default:
+		return nativePath
+	}
+}
+
+// BuildHeaders 构建请求头
+func (a *Adapter) BuildHeaders(provider *conversion.TargetProvider) map[string]string {
+	headers := map[string]string{
+		"Authorization": "Bearer " + provider.APIKey,
+		"Content-Type":  "application/json",
+	}
+	if org, ok := provider.AdapterConfig["organization"].(string); ok && org != "" {
+		headers["OpenAI-Organization"] = org
+	}
+	return headers
+}
+
+// SupportsInterface 检查是否支持接口类型
+func (a *Adapter) SupportsInterface(interfaceType conversion.InterfaceType) bool {
+	switch interfaceType {
+	case conversion.InterfaceTypeChat,
+		conversion.InterfaceTypeModels,
+		conversion.InterfaceTypeModelInfo,
+		conversion.InterfaceTypeEmbeddings,
+		conversion.InterfaceTypeRerank:
+		return true
+	default:
+		return false
+	}
+}
+
+// DecodeRequest 解码请求
+func (a *Adapter) DecodeRequest(raw []byte) (*canonical.CanonicalRequest, error) {
+	return decodeRequest(raw)
+}
+
+// EncodeRequest 编码请求
+func (a *Adapter) EncodeRequest(req *canonical.CanonicalRequest, provider *conversion.TargetProvider) ([]byte, error) {
+	return encodeRequest(req, provider)
+}
+
+// DecodeResponse 解码响应
+func (a *Adapter) DecodeResponse(raw []byte) (*canonical.CanonicalResponse, error) {
+	return decodeResponse(raw)
+}
+
+// EncodeResponse 编码响应
+func (a *Adapter) EncodeResponse(resp *canonical.CanonicalResponse) ([]byte, error) {
+	return encodeResponse(resp)
+}
+
+// CreateStreamDecoder 创建流式解码器
+func (a *Adapter) CreateStreamDecoder() conversion.StreamDecoder {
+	return NewStreamDecoder()
+}
+
+// CreateStreamEncoder 创建流式编码器
+func (a *Adapter) CreateStreamEncoder() conversion.StreamEncoder {
+	return NewStreamEncoder()
+}
+
+// EncodeError 编码错误
+func (a *Adapter) EncodeError(err *conversion.ConversionError) ([]byte, int) {
+	errType := mapErrorCode(err.Code)
+	statusCode := 500
+
+	errMsg := ErrorResponse{
+		Error: ErrorDetail{
+			Message: err.Message,
+			Type:    errType,
+			Param:   nil,
+			Code:    string(err.Code),
+		},
+	}
+	body, _ := json.Marshal(errMsg)
+	return body, statusCode
+}
+
+// mapErrorCode 映射错误码到 OpenAI 错误类型
+func mapErrorCode(code conversion.ErrorCode) string {
+	switch code {
+	case conversion.ErrorCodeInvalidInput,
+		conversion.ErrorCodeMissingRequiredField,
+		conversion.ErrorCodeIncompatibleFeature,
+		conversion.ErrorCodeToolCallParseError,
+		conversion.ErrorCodeJSONParseError,
+		conversion.ErrorCodeProtocolConstraint,
+		conversion.ErrorCodeFieldMappingFailure:
+		return "invalid_request_error"
+	default:
+		return "server_error"
+	}
+}
+
+// DecodeModelsResponse 解码模型列表响应
+func (a *Adapter) DecodeModelsResponse(raw []byte) (*canonical.CanonicalModelList, error) {
+	return decodeModelsResponse(raw)
+}
+
+// EncodeModelsResponse 编码模型列表响应
+func (a *Adapter) EncodeModelsResponse(list *canonical.CanonicalModelList) ([]byte, error) {
+	return encodeModelsResponse(list)
+}
+
+// DecodeModelInfoResponse 解码模型详情响应
+func (a *Adapter) DecodeModelInfoResponse(raw []byte) (*canonical.CanonicalModelInfo, error) {
+	return decodeModelInfoResponse(raw)
+}
+
+// EncodeModelInfoResponse 编码模型详情响应
+func (a *Adapter) EncodeModelInfoResponse(info *canonical.CanonicalModelInfo) ([]byte, error) {
+	return encodeModelInfoResponse(info)
+}
+
+// DecodeEmbeddingRequest 解码嵌入请求
+func (a *Adapter) DecodeEmbeddingRequest(raw []byte) (*canonical.CanonicalEmbeddingRequest, error) {
+	return decodeEmbeddingRequest(raw)
+}
+
+// EncodeEmbeddingRequest 编码嵌入请求
+func (a *Adapter) EncodeEmbeddingRequest(req *canonical.CanonicalEmbeddingRequest, provider *conversion.TargetProvider) ([]byte, error) {
+	return encodeEmbeddingRequest(req, provider)
+}
+
+// DecodeEmbeddingResponse 解码嵌入响应
+func (a *Adapter) DecodeEmbeddingResponse(raw []byte) (*canonical.CanonicalEmbeddingResponse, error) {
+	return decodeEmbeddingResponse(raw)
+}
+
+// EncodeEmbeddingResponse 编码嵌入响应
+func (a *Adapter) EncodeEmbeddingResponse(resp *canonical.CanonicalEmbeddingResponse) ([]byte, error) {
+	return encodeEmbeddingResponse(resp)
+}
+
+// DecodeRerankRequest 解码重排序请求
+func (a *Adapter) DecodeRerankRequest(raw []byte) (*canonical.CanonicalRerankRequest, error) {
+	return decodeRerankRequest(raw)
+}
+
+// EncodeRerankRequest 编码重排序请求
+func (a *Adapter) EncodeRerankRequest(req *canonical.CanonicalRerankRequest, provider *conversion.TargetProvider) ([]byte, error) {
+	return encodeRerankRequest(req, provider)
+}
+
+// DecodeRerankResponse 解码重排序响应
+func (a *Adapter) DecodeRerankResponse(raw []byte) (*canonical.CanonicalRerankResponse, error) {
+	return decodeRerankResponse(raw)
+}
+
+// EncodeRerankResponse 编码重排序响应
+func (a *Adapter) EncodeRerankResponse(resp *canonical.CanonicalRerankResponse) ([]byte, error) {
+	return encodeRerankResponse(resp)
+}