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/protocol/anthropic/stream_converter.go

@@ -0,0 +1,164 @@
+package anthropic
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"nex/backend/internal/protocol/openai"
+)
+
+// StreamConverter 流式转换器
+type StreamConverter struct {
+	messageID    string
+	model        string
+	index        int // 当前 content block index
+	toolCallArgs map[int]string // 缓存每个 tool_call 的 arguments
+	sentStart    bool // 是否已发送 message_start
+	sentBlockStart map[int]bool // 每个 index 是否已发送 content_block_start
+}
+
+// NewStreamConverter 创建流式转换器
+func NewStreamConverter(messageID, model string) *StreamConverter {
+	return &StreamConverter{
+		messageID:      messageID,
+		model:          model,
+		index:          0,
+		toolCallArgs:   make(map[int]string),
+		sentStart:      false,
+		sentBlockStart: make(map[int]bool),
+	}
+}
+
+// ConvertChunk 转换 OpenAI 流块为 Anthropic 事件
+func (c *StreamConverter) ConvertChunk(chunk *openai.StreamChunk) ([]StreamEvent, error) {
+	var events []StreamEvent
+
+	// 发送 message_start（仅一次）
+	if !c.sentStart {
+		events = append(events, StreamEvent{
+			Type: "message_start",
+			Message: &MessagesResponse{
+				ID:    c.messageID,
+				Type:  "message",
+				Role:  "assistant",
+				Model: c.model,
+				Content: []ContentBlock{},
+				Usage: Usage{
+					InputTokens:  0,
+					OutputTokens: 0,
+				},
+			},
+		})
+		c.sentStart = true
+	}
+
+	// 处理每个 choice
+	for _, choice := range chunk.Choices {
+		// 处理 content delta
+		if choice.Delta.Content != "" {
+			// 发送 content_block_start（如果还没发送）
+			if !c.sentBlockStart[c.index] {
+				events = append(events, StreamEvent{
+					Type:  "content_block_start",
+					Index: c.index,
+					ContentBlock: &ContentBlock{
+						Type: "text",
+					},
+				})
+				c.sentBlockStart[c.index] = true
+			}
+
+			// 发送 text delta
+			events = append(events, StreamEvent{
+				Type:  "content_block_delta",
+				Index: c.index,
+				Delta: &Delta{
+					Type: "text_delta",
+					Text: choice.Delta.Content,
+				},
+			})
+		}
+
+		// 处理 tool_calls delta
+		if len(choice.Delta.ToolCalls) > 0 {
+			for _, tc := range choice.Delta.ToolCalls {
+				// 确定 tool_call index
+				toolIndex := c.index + len(c.toolCallArgs)
+
+				// 发送 content_block_start（如果还没发送）
+				if !c.sentBlockStart[toolIndex] {
+					events = append(events, StreamEvent{
+						Type:  "content_block_start",
+						Index: toolIndex,
+						ContentBlock: &ContentBlock{
+							Type: "tool_use",
+							ID:   tc.ID,
+							Name: tc.Function.Name,
+						},
+					})
+					c.sentBlockStart[toolIndex] = true
+					c.toolCallArgs[toolIndex] = ""
+				}
+
+				// 缓存 arguments
+				c.toolCallArgs[toolIndex] += tc.Function.Arguments
+
+				// 发送 input delta
+				events = append(events, StreamEvent{
+					Type:  "content_block_delta",
+					Index: toolIndex,
+					Delta: &Delta{
+						Type:  "input_json_delta",
+						Input: tc.Function.Arguments,
+					},
+				})
+			}
+		}
+
+		// 处理 finish_reason
+		if choice.FinishReason != "" {
+			// 发送 content_block_stop
+			for idx := range c.sentBlockStart {
+				events = append(events, StreamEvent{
+					Type:  "content_block_stop",
+					Index: idx,
+				})
+			}
+
+			// 转换 stop_reason
+			stopReason := ""
+			switch choice.FinishReason {
+			case "stop":
+				stopReason = "end_turn"
+			case "tool_calls":
+				stopReason = "tool_use"
+			case "length":
+				stopReason = "max_tokens"
+			}
+
+			// 发送 message_delta
+			events = append(events, StreamEvent{
+				Type: "message_delta",
+				Delta: &Delta{
+					StopReason: stopReason,
+				},
+			})
+
+			// 发送 message_stop
+			events = append(events, StreamEvent{
+				Type: "message_stop",
+			})
+		}
+	}
+
+	return events, nil
+}
+
+// SerializeEvent 序列化事件为 SSE 格式
+func SerializeEvent(event StreamEvent) (string, error) {
+	bytes, err := json.Marshal(event)
+	if err != nil {
+		return "", err
+	}
+	return fmt.Sprintf("event: %s\ndata: %s\n\n", event.Type, string(bytes)), nil
+}