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/handler/handler_test.go

@@ -4,7 +4,9 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"fmt"
 	"net/http/httptest"
+	"strings"
 	"testing"
 	"time"
 
@@ -13,7 +15,6 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"nex/backend/internal/domain"
-	"nex/backend/internal/protocol/openai"
 	"nex/backend/internal/provider"
 	appErrors "nex/backend/pkg/errors"
 )
@@ -34,8 +35,8 @@ func (m *mockRoutingService) Route(modelName string) (*domain.RouteResult, error
 }
 
 type mockStatsService struct {
-	err      error
-	stats    []domain.UsageStats
+	err        error
+	stats      []domain.UsageStats
 	aggrResult []map[string]interface{}
 }
 
@@ -84,61 +85,14 @@ func (m *mockModelService) Update(id string, updates map[string]interface{}) err
 func (m *mockModelService) Delete(id string) error { return m.err }
 
 type mockProviderClient struct {
-	resp      *openai.ChatCompletionResponse
-	eventChan chan provider.StreamEvent
-	err       error
+	err error
 }
 
-func (m *mockProviderClient) SendRequest(ctx context.Context, req *openai.ChatCompletionRequest, apiKey, baseURL string) (*openai.ChatCompletionResponse, error) {
-	return m.resp, m.err
+func (m *mockProviderClient) Send(ctx context.Context, spec interface{}) (interface{}, error) {
+	return nil, m.err
 }
-func (m *mockProviderClient) SendStreamRequest(ctx context.Context, req *openai.ChatCompletionRequest, apiKey, baseURL string) (<-chan provider.StreamEvent, error) {
-	return m.eventChan, m.err
-}
-
-// ============ OpenAI Handler 测试 ============
-
-func TestOpenAIHandler_HandleChatCompletions_InvalidJSON(t *testing.T) {
-	h := NewOpenAIHandler(nil, nil, nil)
-	w := httptest.NewRecorder()
-	c, _ := gin.CreateTestContext(w)
-	c.Request = httptest.NewRequest("POST", "/v1/chat/completions", bytes.NewReader([]byte("invalid")))
-
-	h.HandleChatCompletions(c)
-	assert.Equal(t, 400, w.Code)
-}
-
-func TestOpenAIHandler_HandleChatCompletions_ValidationError(t *testing.T) {
-	h := NewOpenAIHandler(nil, nil, nil)
-
-	// 缺少 model 字段
-	body, _ := json.Marshal(map[string]interface{}{
-		"messages": []map[string]string{{"role": "user", "content": "hi"}},
-	})
-	w := httptest.NewRecorder()
-	c, _ := gin.CreateTestContext(w)
-	c.Request = httptest.NewRequest("POST", "/v1/chat/completions", bytes.NewReader(body))
-	c.Request.Header.Set("Content-Type", "application/json")
-
-	h.HandleChatCompletions(c)
-	assert.Equal(t, 400, w.Code)
-}
-
-func TestOpenAIHandler_HandleChatCompletions_RouteError(t *testing.T) {
-	routingSvc := &mockRoutingService{err: appErrors.ErrModelNotFound}
-	h := NewOpenAIHandler(nil, routingSvc, nil)
-
-	body, _ := json.Marshal(map[string]interface{}{
-		"model":    "nonexistent",
-		"messages": []map[string]string{{"role": "user", "content": "hi"}},
-	})
-	w := httptest.NewRecorder()
-	c, _ := gin.CreateTestContext(w)
-	c.Request = httptest.NewRequest("POST", "/v1/chat/completions", bytes.NewReader(body))
-	c.Request.Header.Set("Content-Type", "application/json")
-
-	h.HandleChatCompletions(c)
-	assert.Equal(t, 404, w.Code)
+func (m *mockProviderClient) SendStream(ctx context.Context, spec interface{}) (<-chan provider.StreamEvent, error) {
+	return nil, m.err
 }
 
 // ============ Provider Handler 测试 ============
@@ -283,8 +237,16 @@ func TestFormatValidationErrors(t *testing.T) {
 		"model":    "模型名称不能为空",
 		"messages": "消息列表不能为空",
 	}
-	result := formatValidationErrors(errs)
+	result := formatMapErrors(errs)
 	require.Contains(t, result, "请求验证失败")
 	require.Contains(t, result, "model")
 	require.Contains(t, result, "messages")
 }
+
+func formatMapErrors(errs map[string]string) string {
+	parts := make([]string, 0, len(errs))
+	for field, msg := range errs {
+		parts = append(parts, fmt.Sprintf("%s: %s", field, msg))
+	}
+	return "请求验证失败: " + strings.Join(parts, "; ")
+}