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/errors.go

@@ -0,0 +1,83 @@
+package conversion
+
+import "fmt"
+
+// ErrorCode 错误码枚举
+type ErrorCode string
+
+const (
+	ErrorCodeInvalidInput            ErrorCode = "INVALID_INPUT"
+	ErrorCodeMissingRequiredField    ErrorCode = "MISSING_REQUIRED_FIELD"
+	ErrorCodeIncompatibleFeature     ErrorCode = "INCOMPATIBLE_FEATURE"
+	ErrorCodeFieldMappingFailure     ErrorCode = "FIELD_MAPPING_FAILURE"
+	ErrorCodeToolCallParseError      ErrorCode = "TOOL_CALL_PARSE_ERROR"
+	ErrorCodeJSONParseError          ErrorCode = "JSON_PARSE_ERROR"
+	ErrorCodeStreamStateError        ErrorCode = "STREAM_STATE_ERROR"
+	ErrorCodeUTF8DecodeError         ErrorCode = "UTF8_DECODE_ERROR"
+	ErrorCodeProtocolConstraint      ErrorCode = "PROTOCOL_CONSTRAINT_VIOLATION"
+	ErrorCodeEncodingFailure         ErrorCode = "ENCODING_FAILURE"
+	ErrorCodeInterfaceNotSupported   ErrorCode = "INTERFACE_NOT_SUPPORTED"
+)
+
+// ConversionError 协议转换错误
+type ConversionError struct {
+	Code             ErrorCode
+	Message          string
+	ClientProtocol   string
+	ProviderProtocol string
+	InterfaceType    string
+	Details          map[string]any
+	Cause            error
+}
+
+// NewConversionError 创建转换错误
+func NewConversionError(code ErrorCode, message string) *ConversionError {
+	return &ConversionError{
+		Code:    code,
+		Message: message,
+		Details: make(map[string]any),
+	}
+}
+
+// WithClientProtocol 设置客户端协议
+func (e *ConversionError) WithClientProtocol(protocol string) *ConversionError {
+	e.ClientProtocol = protocol
+	return e
+}
+
+// WithProviderProtocol 设置服务端协议
+func (e *ConversionError) WithProviderProtocol(protocol string) *ConversionError {
+	e.ProviderProtocol = protocol
+	return e
+}
+
+// WithInterfaceType 设置接口类型
+func (e *ConversionError) WithInterfaceType(ifaceType string) *ConversionError {
+	e.InterfaceType = ifaceType
+	return e
+}
+
+// WithDetail 添加详情
+func (e *ConversionError) WithDetail(key string, value any) *ConversionError {
+	e.Details[key] = value
+	return e
+}
+
+// WithCause 设置原因
+func (e *ConversionError) WithCause(cause error) *ConversionError {
+	e.Cause = cause
+	return e
+}
+
+// Error 实现 error 接口
+func (e *ConversionError) Error() string {
+	if e.Cause != nil {
+		return fmt.Sprintf("[%s] %s: %v", e.Code, e.Message, e.Cause)
+	}
+	return fmt.Sprintf("[%s] %s", e.Code, e.Message)
+}
+
+// Unwrap 支持 errors.Is/As
+func (e *ConversionError) Unwrap() error {
+	return e.Cause
+}