fix: 修正 conversion 代理路径和错误边界

backend/internal/conversion/errors.go

@@ -17,6 +17,13 @@ const (
 	ErrorCodeProtocolConstraint    ErrorCode = "PROTOCOL_CONSTRAINT_VIOLATION"
 	ErrorCodeEncodingFailure       ErrorCode = "ENCODING_FAILURE"
 	ErrorCodeInterfaceNotSupported ErrorCode = "INTERFACE_NOT_SUPPORTED"
+	ErrorCodeUnsupportedMultimodal ErrorCode = "UNSUPPORTED_MULTIMODAL"
+)
+
+const (
+	ErrorDetailPhase   = "phase"
+	ErrorPhaseRequest  = "request"
+	ErrorPhaseResponse = "response"
 )
 
 // ConversionError 协议转换错误
@@ -39,6 +46,20 @@ func NewConversionError(code ErrorCode, message string) *ConversionError {
 	}
 }
 
+// NewRequestJSONParseError 创建请求 JSON 解析错误。
+func NewRequestJSONParseError(message string, cause error) *ConversionError {
+	return NewConversionError(ErrorCodeJSONParseError, message).
+		WithDetail(ErrorDetailPhase, ErrorPhaseRequest).
+		WithCause(cause)
+}
+
+// NewResponseJSONParseError 创建响应 JSON 解析错误。
+func NewResponseJSONParseError(message string, cause error) *ConversionError {
+	return NewConversionError(ErrorCodeJSONParseError, message).
+		WithDetail(ErrorDetailPhase, ErrorPhaseResponse).
+		WithCause(cause)
+}
+
 // WithClientProtocol 设置客户端协议
 func (e *ConversionError) WithClientProtocol(protocol string) *ConversionError {
 	e.ClientProtocol = protocol