feat(gateway): 增加 SUGGESTION MODE 请求拦截

扩展现有的预热请求拦截功能，新增对 SUGGESTION MODE 请求的拦截：
- 检测 messages 最后一条 user 消息是否以 [SUGGESTION MODE: 开头
- 拦截后返回空内容响应，节省 token 消耗
- 重构检测逻辑，合并为单一函数，只解析一次 JSON

backend/internal/handler/gateway_handler.go

@@ -209,18 +209,21 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 			account := selection.Account
 			setOpsSelectedAccount(c, account.ID)
 
-			// 检查预热请求拦截（在账号选择后、转发前检查）
-			if account.IsInterceptWarmupEnabled() && isWarmupRequest(body) {
+			// 检查请求拦截（预热请求、SUGGESTION MODE等）
+			if account.IsInterceptWarmupEnabled() {
+				interceptType := detectInterceptType(body)
+				if interceptType != InterceptTypeNone {
 					if selection.Acquired && selection.ReleaseFunc != nil {
 						selection.ReleaseFunc()
 					}
 					if reqStream {
-					sendMockWarmupStream(c, reqModel)
+						sendMockInterceptStream(c, reqModel, interceptType)
 					} else {
-					sendMockWarmupResponse(c, reqModel)
+						sendMockInterceptResponse(c, reqModel, interceptType)
 					}
 					return
 				}
+			}
 
 			// 3. 获取账号并发槽位
 			accountReleaseFunc := selection.ReleaseFunc
@@ -344,18 +347,21 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 		account := selection.Account
 		setOpsSelectedAccount(c, account.ID)
 
-		// 检查预热请求拦截（在账号选择后、转发前检查）
-		if account.IsInterceptWarmupEnabled() && isWarmupRequest(body) {
+		// 检查请求拦截（预热请求、SUGGESTION MODE等）
+		if account.IsInterceptWarmupEnabled() {
+			interceptType := detectInterceptType(body)
+			if interceptType != InterceptTypeNone {
 				if selection.Acquired && selection.ReleaseFunc != nil {
 					selection.ReleaseFunc()
 				}
 				if reqStream {
-				sendMockWarmupStream(c, reqModel)
+					sendMockInterceptStream(c, reqModel, interceptType)
 				} else {
-				sendMockWarmupResponse(c, reqModel)
+					sendMockInterceptResponse(c, reqModel, interceptType)
 				}
 				return
 			}
+		}
 
 		// 3. 获取账号并发槽位
 		accountReleaseFunc := selection.ReleaseFunc
@@ -765,17 +771,30 @@ func (h *GatewayHandler) CountTokens(c *gin.Context) {
 	}
 }
 
-// isWarmupRequest 检测是否为预热请求（标题生成、Warmup等）
-func isWarmupRequest(body []byte) bool {
-	// 快速检查：如果body不包含关键字，直接返回false
+// InterceptType 表示请求拦截类型
+type InterceptType int
+
+const (
+	InterceptTypeNone           InterceptType = iota
+	InterceptTypeWarmup                       // 预热请求（返回 "New Conversation"）
+	InterceptTypeSuggestionMode               // SUGGESTION MODE（返回空字符串）
+)
+
+// detectInterceptType 检测请求是否需要拦截，返回拦截类型
+func detectInterceptType(body []byte) InterceptType {
+	// 快速检查：如果不包含任何关键字，直接返回
 	bodyStr := string(body)
-	if !strings.Contains(bodyStr, "title") && !strings.Contains(bodyStr, "Warmup") {
-		return false
+	hasSuggestionMode := strings.Contains(bodyStr, "[SUGGESTION MODE:")
+	hasWarmupKeyword := strings.Contains(bodyStr, "title") || strings.Contains(bodyStr, "Warmup")
+
+	if !hasSuggestionMode && !hasWarmupKeyword {
+		return InterceptTypeNone
 	}
 
-	// 解析完整请求
+	// 解析请求（只解析一次）
 	var req struct {
 		Messages []struct {
+			Role    string `json:"role"`
 			Content []struct {
 				Type string `json:"type"`
 				Text string `json:"text"`
@@ -786,43 +805,71 @@ func isWarmupRequest(body []byte) bool {
 		} `json:"system"`
 	}
 	if err := json.Unmarshal(body, &req); err != nil {
-		return false
+		return InterceptTypeNone
 	}
 
+	// 检查 SUGGESTION MODE（最后一条 user 消息）
+	if hasSuggestionMode && len(req.Messages) > 0 {
+		lastMsg := req.Messages[len(req.Messages)-1]
+		if lastMsg.Role == "user" && len(lastMsg.Content) > 0 &&
+			lastMsg.Content[0].Type == "text" &&
+			strings.HasPrefix(lastMsg.Content[0].Text, "[SUGGESTION MODE:") {
+			return InterceptTypeSuggestionMode
+		}
+	}
+
+	// 检查 Warmup 请求
+	if hasWarmupKeyword {
 		// 检查 messages 中的标题提示模式
 		for _, msg := range req.Messages {
 			for _, content := range msg.Content {
 				if content.Type == "text" {
 					if strings.Contains(content.Text, "Please write a 5-10 word title for the following conversation:") ||
 						content.Text == "Warmup" {
-					return true
+						return InterceptTypeWarmup
 					}
 				}
 			}
 		}
-
 		// 检查 system 中的标题提取模式
-	for _, system := range req.System {
-		if strings.Contains(system.Text, "nalyze if this message indicates a new conversation topic. If it does, extract a 2-3 word title") {
-			return true
+		for _, sys := range req.System {
+			if strings.Contains(sys.Text, "nalyze if this message indicates a new conversation topic. If it does, extract a 2-3 word title") {
+				return InterceptTypeWarmup
+			}
 		}
 	}
 
-	return false
+	return InterceptTypeNone
 }
 
-// sendMockWarmupStream 发送流式 mock 响应（用于预热请求拦截）
-func sendMockWarmupStream(c *gin.Context, model string) {
+// sendMockInterceptStream 发送流式 mock 响应（用于请求拦截）
+func sendMockInterceptStream(c *gin.Context, model string, interceptType InterceptType) {
 	c.Header("Content-Type", "text/event-stream")
 	c.Header("Cache-Control", "no-cache")
 	c.Header("Connection", "keep-alive")
 	c.Header("X-Accel-Buffering", "no")
 
+	// 根据拦截类型决定响应内容
+	var msgID string
+	var outputTokens int
+	var textDeltas []string
+
+	switch interceptType {
+	case InterceptTypeSuggestionMode:
+		msgID = "msg_mock_suggestion"
+		outputTokens = 1
+		textDeltas = []string{""} // 空内容
+	default: // InterceptTypeWarmup
+		msgID = "msg_mock_warmup"
+		outputTokens = 2
+		textDeltas = []string{"New", " Conversation"}
+	}
+
 	// Build message_start event with proper JSON marshaling
 	messageStart := map[string]any{
 		"type": "message_start",
 		"message": map[string]any{
-			"id":            "msg_mock_warmup",
+			"id":            msgID,
 			"type":          "message",
 			"role":          "assistant",
 			"model":         model,
@@ -837,16 +884,46 @@ func sendMockWarmupStream(c *gin.Context, model string) {
 	}
 	messageStartJSON, _ := json.Marshal(messageStart)
 
+	// Build events
 	events := []string{
 		`event: message_start` + "\n" + `data: ` + string(messageStartJSON),
 		`event: content_block_start` + "\n" + `data: {"content_block":{"text":"","type":"text"},"index":0,"type":"content_block_start"}`,
-		`event: content_block_delta` + "\n" + `data: {"delta":{"text":"New","type":"text_delta"},"index":0,"type":"content_block_delta"}`,
-		`event: content_block_delta` + "\n" + `data: {"delta":{"text":" Conversation","type":"text_delta"},"index":0,"type":"content_block_delta"}`,
-		`event: content_block_stop` + "\n" + `data: {"index":0,"type":"content_block_stop"}`,
-		`event: message_delta` + "\n" + `data: {"delta":{"stop_reason":"end_turn","stop_sequence":null},"type":"message_delta","usage":{"input_tokens":10,"output_tokens":2}}`,
-		`event: message_stop` + "\n" + `data: {"type":"message_stop"}`,
 	}
 
+	// Add text deltas
+	for _, text := range textDeltas {
+		delta := map[string]any{
+			"type":  "content_block_delta",
+			"index": 0,
+			"delta": map[string]string{
+				"type": "text_delta",
+				"text": text,
+			},
+		}
+		deltaJSON, _ := json.Marshal(delta)
+		events = append(events, `event: content_block_delta`+"\n"+`data: `+string(deltaJSON))
+	}
+
+	// Add final events
+	messageDelta := map[string]any{
+		"type": "message_delta",
+		"delta": map[string]any{
+			"stop_reason":   "end_turn",
+			"stop_sequence": nil,
+		},
+		"usage": map[string]int{
+			"input_tokens":  10,
+			"output_tokens": outputTokens,
+		},
+	}
+	messageDeltaJSON, _ := json.Marshal(messageDelta)
+
+	events = append(events,
+		`event: content_block_stop`+"\n"+`data: {"index":0,"type":"content_block_stop"}`,
+		`event: message_delta`+"\n"+`data: `+string(messageDeltaJSON),
+		`event: message_stop`+"\n"+`data: {"type":"message_stop"}`,
+	)
+
 	for _, event := range events {
 		_, _ = c.Writer.WriteString(event + "\n\n")
 		c.Writer.Flush()
@@ -854,18 +931,32 @@ func sendMockWarmupStream(c *gin.Context, model string) {
 	}
 }
 
-// sendMockWarmupResponse 发送非流式 mock 响应（用于预热请求拦截）
-func sendMockWarmupResponse(c *gin.Context, model string) {
+// sendMockInterceptResponse 发送非流式 mock 响应（用于请求拦截）
+func sendMockInterceptResponse(c *gin.Context, model string, interceptType InterceptType) {
+	var msgID, text string
+	var outputTokens int
+
+	switch interceptType {
+	case InterceptTypeSuggestionMode:
+		msgID = "msg_mock_suggestion"
+		text = ""
+		outputTokens = 1
+	default: // InterceptTypeWarmup
+		msgID = "msg_mock_warmup"
+		text = "New Conversation"
+		outputTokens = 2
+	}
+
 	c.JSON(http.StatusOK, gin.H{
-		"id":          "msg_mock_warmup",
+		"id":          msgID,
 		"type":        "message",
 		"role":        "assistant",
 		"model":       model,
-		"content":     []gin.H{{"type": "text", "text": "New Conversation"}},
+		"content":     []gin.H{{"type": "text", "text": text}},
 		"stop_reason": "end_turn",
 		"usage": gin.H{
 			"input_tokens":  10,
-			"output_tokens": 2,
+			"output_tokens": outputTokens,
 		},
 	})
 }