refactor: 实现 ConversionEngine 协议转换引擎，替代旧 protocol 包

- 新增 ConversionEngine 核心引擎，支持 OpenAI 和 Anthropic 协议转换
- 添加 stream decoder/encoder 实现
- 更新 provider client 支持新引擎
- 补充单元测试和集成测试
- 更新 specs 文档

backend/internal/service/service_supplemental_test.go

@@ -0,0 +1,154 @@
+package service
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"nex/backend/internal/domain"
+	"nex/backend/internal/repository"
+)
+
+func TestProviderService_Update(t *testing.T) {
+	db := setupServiceTestDB(t)
+	repo := repository.NewProviderRepository(db)
+	svc := NewProviderService(repo)
+
+	svc.Create(&domain.Provider{ID: "p1", Name: "Original", APIKey: "key", BaseURL: "https://test.com"})
+
+	err := svc.Update("p1", map[string]interface{}{"name": "Updated"})
+	require.NoError(t, err)
+
+	result, err := svc.Get("p1", false)
+	require.NoError(t, err)
+	assert.Equal(t, "Updated", result.Name)
+}
+
+func TestProviderService_Update_NotFound(t *testing.T) {
+	db := setupServiceTestDB(t)
+	repo := repository.NewProviderRepository(db)
+	svc := NewProviderService(repo)
+
+	err := svc.Update("nonexistent", map[string]interface{}{"name": "test"})
+	assert.Error(t, err)
+}
+
+func TestModelService_Get(t *testing.T) {
+	db := setupServiceTestDB(t)
+	providerRepo := repository.NewProviderRepository(db)
+	modelRepo := repository.NewModelRepository(db)
+	svc := NewModelService(modelRepo, providerRepo)
+
+	providerRepo.Create(&domain.Provider{ID: "p1", Name: "P1", APIKey: "key", BaseURL: "https://test.com"})
+	svc.Create(&domain.Model{ID: "m1", ProviderID: "p1", ModelName: "gpt-4"})
+
+	model, err := svc.Get("m1")
+	require.NoError(t, err)
+	assert.Equal(t, "gpt-4", model.ModelName)
+}
+
+func TestModelService_Update(t *testing.T) {
+	db := setupServiceTestDB(t)
+	providerRepo := repository.NewProviderRepository(db)
+	modelRepo := repository.NewModelRepository(db)
+	svc := NewModelService(modelRepo, providerRepo)
+
+	providerRepo.Create(&domain.Provider{ID: "p1", Name: "P1", APIKey: "key", BaseURL: "https://test.com"})
+	svc.Create(&domain.Model{ID: "m1", ProviderID: "p1", ModelName: "gpt-4"})
+
+	err := svc.Update("m1", map[string]interface{}{"model_name": "gpt-4o"})
+	require.NoError(t, err)
+
+	model, err := svc.Get("m1")
+	require.NoError(t, err)
+	assert.Equal(t, "gpt-4o", model.ModelName)
+}
+
+func TestModelService_Update_ProviderID_Invalid(t *testing.T) {
+	db := setupServiceTestDB(t)
+	providerRepo := repository.NewProviderRepository(db)
+	modelRepo := repository.NewModelRepository(db)
+	svc := NewModelService(modelRepo, providerRepo)
+
+	providerRepo.Create(&domain.Provider{ID: "p1", Name: "P1", APIKey: "key", BaseURL: "https://test.com"})
+	svc.Create(&domain.Model{ID: "m1", ProviderID: "p1", ModelName: "gpt-4"})
+
+	err := svc.Update("m1", map[string]interface{}{"provider_id": "nonexistent"})
+	assert.Error(t, err)
+}
+
+func TestModelService_Delete(t *testing.T) {
+	db := setupServiceTestDB(t)
+	providerRepo := repository.NewProviderRepository(db)
+	modelRepo := repository.NewModelRepository(db)
+	svc := NewModelService(modelRepo, providerRepo)
+
+	providerRepo.Create(&domain.Provider{ID: "p1", Name: "P1", APIKey: "key", BaseURL: "https://test.com"})
+	svc.Create(&domain.Model{ID: "m1", ProviderID: "p1", ModelName: "gpt-4"})
+
+	err := svc.Delete("m1")
+	require.NoError(t, err)
+
+	_, err = svc.Get("m1")
+	assert.Error(t, err)
+}
+
+func TestModelService_Delete_NotFound(t *testing.T) {
+	db := setupServiceTestDB(t)
+	modelRepo := repository.NewModelRepository(db)
+	providerRepo := repository.NewProviderRepository(db)
+	svc := NewModelService(modelRepo, providerRepo)
+
+	err := svc.Delete("nonexistent")
+	assert.Error(t, err)
+}
+
+func TestStatsService_Aggregate_ByModel(t *testing.T) {
+	statsRepo := repository.NewStatsRepository(nil)
+	svc := NewStatsService(statsRepo)
+
+	stats := []domain.UsageStats{
+		{ProviderID: "p1", ModelName: "gpt-4", RequestCount: 10},
+		{ProviderID: "p1", ModelName: "gpt-4", RequestCount: 5},
+		{ProviderID: "p2", ModelName: "gpt-4", RequestCount: 8},
+	}
+
+	result := svc.Aggregate(stats, "model")
+	assert.True(t, len(result) >= 1)
+
+	totalCount := 0
+	for _, r := range result {
+		totalCount += r["request_count"].(int)
+	}
+	assert.Equal(t, 23, totalCount)
+}
+
+func TestStatsService_Aggregate_Default(t *testing.T) {
+	statsRepo := repository.NewStatsRepository(nil)
+	svc := NewStatsService(statsRepo)
+
+	stats := []domain.UsageStats{
+		{ProviderID: "p1", RequestCount: 10},
+		{ProviderID: "p2", RequestCount: 5},
+	}
+
+	result := svc.Aggregate(stats, "")
+	assert.Len(t, result, 2)
+
+	totalCount := 0
+	for _, r := range result {
+		totalCount += r["request_count"].(int)
+	}
+	assert.Equal(t, 15, totalCount)
+}
+
+func TestModelService_Update_NotFound(t *testing.T) {
+	db := setupServiceTestDB(t)
+	modelRepo := repository.NewModelRepository(db)
+	providerRepo := repository.NewProviderRepository(db)
+	svc := NewModelService(modelRepo, providerRepo)
+
+	err := svc.Update("nonexistent", map[string]interface{}{"model_name": "test"})
+	assert.Error(t, err)
+}