feat: 实现分层架构，包含 domain、service、repository 和 pkg 层

- 新增 domain 层：model、provider、route、stats 实体
- 新增 service 层：models、providers、routing、stats 业务逻辑
- 新增 repository 层：models、providers、stats 数据访问
- 新增 pkg 工具包：errors、logger、validator
- 新增中间件：CORS、logging、recovery、request ID
- 新增数据库迁移：初始 schema 和索引
- 新增单元测试和集成测试
- 新增规范文档：config-management、database-migration、error-handling、layered-architecture、middleware-system、request-validation、structured-logging、test-coverage
- 移除 config 子包和 model_router（已迁移至分层架构）

backend/internal/provider/client_test.go

@@ -0,0 +1,151 @@
+package provider
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"nex/backend/internal/protocol/openai"
+)
+
+func TestNewClient(t *testing.T) {
+	client := NewClient()
+	require.NotNil(t, client)
+	assert.NotNil(t, client.httpClient)
+	assert.NotNil(t, client.adapter)
+	assert.Equal(t, 4096, client.streamCfg.InitialBufferSize)
+	assert.Equal(t, 65536, client.streamCfg.MaxBufferSize)
+	assert.Equal(t, 100, client.streamCfg.ChannelBufferSize)
+}
+
+func TestDefaultStreamConfig(t *testing.T) {
+	cfg := DefaultStreamConfig()
+	assert.Equal(t, 4096, cfg.InitialBufferSize)
+	assert.Equal(t, 65536, cfg.MaxBufferSize)
+	assert.Equal(t, 100, cfg.ChannelBufferSize)
+}
+
+func TestClient_SendRequest_Success(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "POST", r.Method)
+		assert.Equal(t, "application/json", r.Header.Get("Content-Type"))
+		assert.Equal(t, "Bearer test-key", r.Header.Get("Authorization"))
+
+		resp := openai.ChatCompletionResponse{
+			ID: "chatcmpl-123",
+			Choices: []openai.Choice{
+				{Index: 0, Message: &openai.Message{Role: "assistant", Content: "Hello!"}},
+			},
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(resp)
+	}))
+	defer server.Close()
+
+	client := NewClient()
+	req := &openai.ChatCompletionRequest{
+		Model:    "gpt-4",
+		Messages: []openai.Message{{Role: "user", Content: "Hi"}},
+	}
+
+	result, err := client.SendRequest(context.Background(), req, "test-key", server.URL)
+	require.NoError(t, err)
+	assert.Equal(t, "chatcmpl-123", result.ID)
+}
+
+func TestClient_SendRequest_ErrorResponse(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusUnauthorized)
+		json.NewEncoder(w).Encode(openai.ErrorResponse{
+			Error: openai.ErrorDetail{Message: "Invalid API key"},
+		})
+	}))
+	defer server.Close()
+
+	client := NewClient()
+	req := &openai.ChatCompletionRequest{
+		Model:    "gpt-4",
+		Messages: []openai.Message{{Role: "user", Content: "Hi"}},
+	}
+
+	_, err := client.SendRequest(context.Background(), req, "bad-key", server.URL)
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "Invalid API key")
+}
+
+func TestClient_SendRequest_ConnectionError(t *testing.T) {
+	client := NewClient()
+	req := &openai.ChatCompletionRequest{
+		Model:    "gpt-4",
+		Messages: []openai.Message{{Role: "user", Content: "Hi"}},
+	}
+
+	_, err := client.SendRequest(context.Background(), req, "key", "http://localhost:1")
+	assert.Error(t, err)
+}
+
+func TestClient_SendStreamRequest_CreatesChannel(t *testing.T) {
+	// 使用一个慢服务器确保客户端有时间读取
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/event-stream")
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer server.Close()
+
+	client := NewClient()
+	req := &openai.ChatCompletionRequest{
+		Model:    "gpt-4",
+		Messages: []openai.Message{{Role: "user", Content: "Hi"}},
+	}
+
+	eventChan, err := client.SendStreamRequest(context.Background(), req, "test-key", server.URL)
+	require.NoError(t, err)
+	require.NotNil(t, eventChan)
+
+	// 读取直到 channel 关闭（服务器关闭后应产生 EOF）
+	for range eventChan {
+		// 消费所有事件
+	}
+	// channel 应已关闭（不阻塞即通过）
+}
+
+func TestClient_SendStreamRequest_ErrorResponse(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+	}))
+	defer server.Close()
+
+	client := NewClient()
+	req := &openai.ChatCompletionRequest{
+		Model:    "gpt-4",
+		Messages: []openai.Message{{Role: "user", Content: "Hi"}},
+	}
+
+	_, err := client.SendStreamRequest(context.Background(), req, "key", server.URL)
+	assert.Error(t, err)
+}
+
+func TestIsNetworkError(t *testing.T) {
+	tests := []struct {
+		input string
+		want  bool
+	}{
+		{"connection reset by peer", true},
+		{"broken pipe", true},
+		{"network is unreachable", true},
+		{"timeout waiting for response", true},
+		{"unexpected EOF", true},
+		{"normal error", false},
+		{"", false},
+	}
+	for _, tt := range tests {
+		err := fmt.Errorf("%s", tt.input) //nolint:govet
+		assert.Equal(t, tt.want, isNetworkError(err), "isNetworkError(%q)", tt.input)
+	}
+}