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/handler/handler_supplemental_test.go

@@ -0,0 +1,165 @@
+package handler
+
+import (
+	"bytes"
+	"encoding/json"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"nex/backend/internal/domain"
+)
+
+func TestProviderHandler_CreateProvider_Success(t *testing.T) {
+	h := NewProviderHandler(&mockProviderService{})
+
+	body, _ := json.Marshal(map[string]string{
+		"id":      "p1",
+		"name":    "Test",
+		"api_key": "sk-test",
+		"base_url": "https://api.test.com",
+	})
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest("POST", "/api/providers", bytes.NewReader(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	h.CreateProvider(c)
+	assert.Equal(t, 201, w.Code)
+
+	var result domain.Provider
+	require.NoError(t, json.Unmarshal(w.Body.Bytes(), &result))
+	assert.Equal(t, "p1", result.ID)
+	assert.Contains(t, result.APIKey, "***")
+}
+
+func TestProviderHandler_CreateProvider_WithProtocol(t *testing.T) {
+	h := NewProviderHandler(&mockProviderService{})
+
+	body, _ := json.Marshal(map[string]string{
+		"id":       "p1",
+		"name":     "Test",
+		"api_key":  "sk-test",
+		"base_url": "https://api.test.com",
+		"protocol": "anthropic",
+	})
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest("POST", "/api/providers", bytes.NewReader(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	h.CreateProvider(c)
+	assert.Equal(t, 201, w.Code)
+}
+
+func TestProviderHandler_UpdateProvider(t *testing.T) {
+	h := NewProviderHandler(&mockProviderService{
+		provider: &domain.Provider{ID: "p1", Name: "Updated", APIKey: "***"},
+	})
+
+	body, _ := json.Marshal(map[string]string{"name": "Updated"})
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "id", Value: "p1"}}
+	c.Request = httptest.NewRequest("PUT", "/api/providers/p1", bytes.NewReader(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	h.UpdateProvider(c)
+	assert.Equal(t, 200, w.Code)
+}
+
+func TestProviderHandler_UpdateProvider_InvalidBody(t *testing.T) {
+	h := NewProviderHandler(&mockProviderService{})
+
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "id", Value: "p1"}}
+	c.Request = httptest.NewRequest("PUT", "/api/providers/p1", nil)
+
+	h.UpdateProvider(c)
+	assert.Equal(t, 400, w.Code)
+}
+
+func TestProviderHandler_DeleteProvider(t *testing.T) {
+	h := NewProviderHandler(&mockProviderService{})
+
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "id", Value: "p1"}}
+	c.Request = httptest.NewRequest("DELETE", "/api/providers/p1", bytes.NewReader([]byte{}))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	h.DeleteProvider(c)
+	assert.True(t, w.Code == 204 || w.Code == 200)
+}
+
+func TestModelHandler_DeleteModel(t *testing.T) {
+	h := NewModelHandler(&mockModelService{})
+
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "id", Value: "m1"}}
+	c.Request = httptest.NewRequest("DELETE", "/api/models/m1", bytes.NewReader([]byte{}))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	h.DeleteModel(c)
+	assert.True(t, w.Code == 204 || w.Code == 200)
+}
+
+func TestModelHandler_CreateModel_Success(t *testing.T) {
+	h := NewModelHandler(&mockModelService{})
+
+	body, _ := json.Marshal(map[string]string{
+		"id":          "m1",
+		"provider_id": "p1",
+		"model_name":  "gpt-4",
+	})
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest("POST", "/api/models", bytes.NewReader(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	h.CreateModel(c)
+	assert.Equal(t, 201, w.Code)
+
+	var result domain.Model
+	require.NoError(t, json.Unmarshal(w.Body.Bytes(), &result))
+	assert.Equal(t, "m1", result.ID)
+}
+
+func TestModelHandler_GetModel(t *testing.T) {
+	h := NewModelHandler(&mockModelService{
+		model: &domain.Model{ID: "m1", ModelName: "gpt-4"},
+	})
+
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "id", Value: "m1"}}
+	c.Request = httptest.NewRequest("GET", "/api/models/m1", nil)
+
+	h.GetModel(c)
+	assert.Equal(t, 200, w.Code)
+
+	var result domain.Model
+	require.NoError(t, json.Unmarshal(w.Body.Bytes(), &result))
+	assert.Equal(t, "gpt-4", result.ModelName)
+}
+
+func TestModelHandler_UpdateModel(t *testing.T) {
+	h := NewModelHandler(&mockModelService{
+		model: &domain.Model{ID: "m1", ModelName: "gpt-4o"},
+	})
+
+	body, _ := json.Marshal(map[string]string{"model_name": "gpt-4o"})
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "id", Value: "m1"}}
+	c.Request = httptest.NewRequest("PUT", "/api/models/m1", bytes.NewReader(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	h.UpdateModel(c)
+	assert.Equal(t, 200, w.Code)
+}