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_test.go

@@ -0,0 +1,45 @@
+package conversion
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestConversionError_Builder(t *testing.T) {
+	cause := errors.New("原始错误")
+	err := NewConversionError(ErrorCodeInvalidInput, "输入无效").
+		WithClientProtocol("openai").
+		WithDetail("field", "model").
+		WithCause(cause)
+
+	assert.Equal(t, ErrorCodeInvalidInput, err.Code)
+	assert.Equal(t, "openai", err.ClientProtocol)
+	assert.Equal(t, "输入无效", err.Message)
+	assert.Equal(t, "model", err.Details["field"])
+	assert.Equal(t, cause, err.Cause)
+}
+
+func TestConversionError_Unwrap(t *testing.T) {
+	cause := errors.New("根本原因")
+	err := NewConversionError(ErrorCodeJSONParseError, "解析失败").WithCause(cause)
+
+	unwrapped := err.Unwrap()
+	assert.Equal(t, cause, unwrapped)
+}
+
+func TestConversionError_Error_WithCause(t *testing.T) {
+	err := NewConversionError(ErrorCodeInvalidInput, "输入无效").WithCause(errors.New("原因"))
+	msg := err.Error()
+	assert.Contains(t, msg, "INVALID_INPUT")
+	assert.Contains(t, msg, "输入无效")
+	assert.Contains(t, msg, "原因")
+}
+
+func TestConversionError_Error_WithoutCause(t *testing.T) {
+	err := NewConversionError(ErrorCodeInvalidInput, "输入无效")
+	msg := err.Error()
+	assert.Contains(t, msg, "INVALID_INPUT")
+	assert.Contains(t, msg, "输入无效")
+}