feat: 初始化 AI Gateway 项目

实现支持 OpenAI 和 Anthropic 双协议的统一大模型 API 网关 MVP 版本，包含：
- OpenAI 和 Anthropic 协议代理
- 供应商和模型管理
- 用量统计
- 前端配置界面

backend/internal/handler/anthropic_handler.go

@@ -0,0 +1,243 @@
+package handler
+
+import (
+	"bufio"
+	"fmt"
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+
+	"nex/backend/internal/config"
+	"nex/backend/internal/protocol/anthropic"
+	"nex/backend/internal/protocol/openai"
+	"nex/backend/internal/provider"
+	"nex/backend/internal/router"
+)
+
+// AnthropicHandler Anthropic 协议处理器
+type AnthropicHandler struct {
+	client *provider.Client
+	router *router.Router
+}
+
+// NewAnthropicHandler 创建 Anthropic 处理器
+func NewAnthropicHandler() *AnthropicHandler {
+	return &AnthropicHandler{
+		client: provider.NewClient(),
+		router: router.NewRouter(),
+	}
+}
+
+// HandleMessages 处理 Messages 请求
+func (h *AnthropicHandler) HandleMessages(c *gin.Context) {
+	// 解析 Anthropic 请求
+	var req anthropic.MessagesRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		c.JSON(http.StatusBadRequest, anthropic.ErrorResponse{
+			Type: "error",
+			Error: anthropic.ErrorDetail{
+				Type:    "invalid_request_error",
+				Message: "无效的请求格式: " + err.Error(),
+			},
+		})
+		return
+	}
+
+	// 检查多模态内容
+	if err := h.checkMultimodalContent(&req); err != nil {
+		c.JSON(http.StatusBadRequest, anthropic.ErrorResponse{
+			Type: "error",
+			Error: anthropic.ErrorDetail{
+				Type:    "invalid_request_error",
+				Message: err.Error(),
+			},
+		})
+		return
+	}
+
+	// 转换为 OpenAI 请求
+	openaiReq, err := anthropic.ConvertRequest(&req)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, anthropic.ErrorResponse{
+			Type: "error",
+			Error: anthropic.ErrorDetail{
+				Type:    "invalid_request_error",
+				Message: "请求转换失败: " + err.Error(),
+			},
+		})
+		return
+	}
+
+	// 路由到供应商
+	routeResult, err := h.router.Route(openaiReq.Model)
+	if err != nil {
+		h.handleError(c, err)
+		return
+	}
+
+	// 根据是否流式选择处理方式
+	if req.Stream {
+		h.handleStreamRequest(c, openaiReq, routeResult)
+	} else {
+		h.handleNonStreamRequest(c, openaiReq, routeResult)
+	}
+}
+
+// handleNonStreamRequest 处理非流式请求
+func (h *AnthropicHandler) handleNonStreamRequest(c *gin.Context, openaiReq *openai.ChatCompletionRequest, routeResult *router.RouteResult) {
+	// 发送请求到供应商
+	openaiResp, err := h.client.SendRequest(c.Request.Context(), openaiReq, routeResult.Provider.APIKey, routeResult.Provider.BaseURL)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, anthropic.ErrorResponse{
+			Type: "error",
+			Error: anthropic.ErrorDetail{
+				Type:    "api_error",
+				Message: "供应商请求失败: " + err.Error(),
+			},
+		})
+		return
+	}
+
+	// 转换为 Anthropic 响应
+	anthropicResp, err := anthropic.ConvertResponse(openaiResp)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, anthropic.ErrorResponse{
+			Type: "error",
+			Error: anthropic.ErrorDetail{
+				Type:    "api_error",
+				Message: "响应转换失败: " + err.Error(),
+			},
+		})
+		return
+	}
+
+	// 记录统计
+	go func() {
+		_ = config.RecordRequest(routeResult.Provider.ID, openaiReq.Model)
+	}()
+
+	// 返回响应
+	c.JSON(http.StatusOK, anthropicResp)
+}
+
+// handleStreamRequest 处理流式请求
+func (h *AnthropicHandler) handleStreamRequest(c *gin.Context, openaiReq *openai.ChatCompletionRequest, routeResult *router.RouteResult) {
+	// 发送流式请求到供应商
+	eventChan, err := h.client.SendStreamRequest(c.Request.Context(), openaiReq, routeResult.Provider.APIKey, routeResult.Provider.BaseURL)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, anthropic.ErrorResponse{
+			Type: "error",
+			Error: anthropic.ErrorDetail{
+				Type:    "api_error",
+				Message: "供应商请求失败: " + err.Error(),
+			},
+		})
+		return
+	}
+
+	// 设置 SSE 响应头
+	c.Header("Content-Type", "text/event-stream")
+	c.Header("Cache-Control", "no-cache")
+	c.Header("Connection", "keep-alive")
+
+	// 创建流写入器
+	writer := bufio.NewWriter(c.Writer)
+
+	// 创建流式转换器
+	converter := anthropic.NewStreamConverter(
+		fmt.Sprintf("msg_%s", routeResult.Provider.ID),
+		openaiReq.Model,
+	)
+
+	// 流式转发事件
+	for event := range eventChan {
+		if event.Error != nil {
+			fmt.Printf("流错误: %v\n", event.Error)
+			break
+		}
+
+		if event.Done {
+			break
+		}
+
+		// 解析 OpenAI 流块
+		chunk, err := openai.NewAdapter().ParseStreamChunk(event.Data)
+		if err != nil {
+			fmt.Printf("解析流块失败: %v\n", err)
+			continue
+		}
+
+		// 转换为 Anthropic 事件
+		anthropicEvents, err := converter.ConvertChunk(chunk)
+		if err != nil {
+			fmt.Printf("转换事件失败: %v\n", err)
+			continue
+		}
+
+		// 写入事件
+		for _, ae := range anthropicEvents {
+			eventStr, err := anthropic.SerializeEvent(ae)
+			if err != nil {
+				fmt.Printf("序列化事件失败: %v\n", err)
+				continue
+			}
+			writer.WriteString(eventStr)
+			writer.Flush()
+		}
+	}
+
+	// 记录统计
+	go func() {
+		_ = config.RecordRequest(routeResult.Provider.ID, openaiReq.Model)
+	}()
+}
+
+// checkMultimodalContent 检查多模态内容
+func (h *AnthropicHandler) checkMultimodalContent(req *anthropic.MessagesRequest) error {
+	for _, msg := range req.Messages {
+		for _, block := range msg.Content {
+			if block.Type == "image" {
+				return fmt.Errorf("MVP 不支持多模态内容（图片）")
+			}
+		}
+	}
+	return nil
+}
+
+// handleError 处理路由错误
+func (h *AnthropicHandler) handleError(c *gin.Context, err error) {
+	switch err {
+	case router.ErrModelNotFound:
+		c.JSON(http.StatusNotFound, anthropic.ErrorResponse{
+			Type: "error",
+			Error: anthropic.ErrorDetail{
+				Type:    "not_found_error",
+				Message: "模型未找到",
+			},
+		})
+	case router.ErrModelDisabled:
+		c.JSON(http.StatusNotFound, anthropic.ErrorResponse{
+			Type: "error",
+			Error: anthropic.ErrorDetail{
+				Type:    "not_found_error",
+				Message: "模型已禁用",
+			},
+		})
+	case router.ErrProviderDisabled:
+		c.JSON(http.StatusNotFound, anthropic.ErrorResponse{
+			Type: "error",
+			Error: anthropic.ErrorDetail{
+				Type:    "not_found_error",
+				Message: "供应商已禁用",
+			},
+		})
+	default:
+		c.JSON(http.StatusInternalServerError, anthropic.ErrorResponse{
+			Type: "error",
+			Error: anthropic.ErrorDetail{
+				Type:    "internal_error",
+				Message: "内部错误: " + err.Error(),
+			},
+		})
+	}
+}