fix(antigravity): add stream keepalive to prevent connection drops

Antigravity streaming handlers were missing the keepalive mechanism
that exists in the standard gateway, causing proxy/CDN idle timeouts
to break connections during long thinking phases (e.g. claude-opus-4-6).
This resulted in truncated responses with missing tool calls.

Add StreamKeepaliveInterval support to all three Antigravity streaming
paths: Claude SSE, Gemini SSE, and upstream passthrough.

backend/internal/service/antigravity_gateway_service.go

@@ -3079,6 +3079,22 @@ func (s *AntigravityGatewayService) handleGeminiStreamingResponse(c *gin.Context
 		intervalCh = intervalTicker.C
 	}
 
+	// 下游 keepalive：防止代理/Cloudflare Tunnel 因连接空闲而断开
+	keepaliveInterval := time.Duration(0)
+	if s.settingService.cfg != nil && s.settingService.cfg.Gateway.StreamKeepaliveInterval > 0 {
+		keepaliveInterval = time.Duration(s.settingService.cfg.Gateway.StreamKeepaliveInterval) * time.Second
+	}
+	var keepaliveTicker *time.Ticker
+	if keepaliveInterval > 0 {
+		keepaliveTicker = time.NewTicker(keepaliveInterval)
+		defer keepaliveTicker.Stop()
+	}
+	var keepaliveCh <-chan time.Time
+	if keepaliveTicker != nil {
+		keepaliveCh = keepaliveTicker.C
+	}
+	lastDataAt := time.Now()
+
 	cw := newAntigravityClientWriter(c.Writer, flusher, "antigravity gemini")
 
 	// 仅发送一次错误事件，避免多次写入导致协议混乱
@@ -3111,6 +3127,8 @@ func (s *AntigravityGatewayService) handleGeminiStreamingResponse(c *gin.Context
 				return nil, ev.err
 			}
 
+			lastDataAt = time.Now()
+
 			line := ev.line
 			trimmed := strings.TrimRight(line, "\r\n")
 			if strings.HasPrefix(trimmed, "data:") {
@@ -3170,6 +3188,19 @@ func (s *AntigravityGatewayService) handleGeminiStreamingResponse(c *gin.Context
 			logger.LegacyPrintf("service.antigravity_gateway", "Stream data interval timeout (antigravity)")
 			sendErrorEvent("stream_timeout")
 			return &antigravityStreamResult{usage: usage, firstTokenMs: firstTokenMs}, fmt.Errorf("stream data interval timeout")
+
+		case <-keepaliveCh:
+			if cw.Disconnected() {
+				continue
+			}
+			if time.Since(lastDataAt) < keepaliveInterval {
+				continue
+			}
+			// SSE ping/keepalive：保持连接活跃防止 Cloudflare Tunnel 等代理断开
+			if !cw.Fprintf(":\n\n") {
+				logger.LegacyPrintf("service.antigravity_gateway", "Client disconnected during keepalive ping (antigravity gemini), continuing to drain upstream for billing")
+				continue
+			}
 		}
 	}
 }
@@ -3895,6 +3926,22 @@ func (s *AntigravityGatewayService) handleClaudeStreamingResponse(c *gin.Context
 		intervalCh = intervalTicker.C
 	}
 
+	// 下游 keepalive：防止代理/Cloudflare Tunnel 因连接空闲而断开
+	keepaliveInterval := time.Duration(0)
+	if s.settingService.cfg != nil && s.settingService.cfg.Gateway.StreamKeepaliveInterval > 0 {
+		keepaliveInterval = time.Duration(s.settingService.cfg.Gateway.StreamKeepaliveInterval) * time.Second
+	}
+	var keepaliveTicker *time.Ticker
+	if keepaliveInterval > 0 {
+		keepaliveTicker = time.NewTicker(keepaliveInterval)
+		defer keepaliveTicker.Stop()
+	}
+	var keepaliveCh <-chan time.Time
+	if keepaliveTicker != nil {
+		keepaliveCh = keepaliveTicker.C
+	}
+	lastDataAt := time.Now()
+
 	cw := newAntigravityClientWriter(c.Writer, flusher, "antigravity claude")
 
 	// 仅发送一次错误事件，避免多次写入导致协议混乱
@@ -3947,6 +3994,8 @@ func (s *AntigravityGatewayService) handleClaudeStreamingResponse(c *gin.Context
 				return nil, fmt.Errorf("stream read error: %w", ev.err)
 			}
 
+			lastDataAt = time.Now()
+
 			// 处理 SSE 行，转换为 Claude 格式
 			claudeEvents := processor.ProcessLine(strings.TrimRight(ev.line, "\r\n"))
 			if len(claudeEvents) > 0 {
@@ -3969,6 +4018,20 @@ func (s *AntigravityGatewayService) handleClaudeStreamingResponse(c *gin.Context
 			logger.LegacyPrintf("service.antigravity_gateway", "Stream data interval timeout (antigravity)")
 			sendErrorEvent("stream_timeout")
 			return &antigravityStreamResult{usage: convertUsage(nil), firstTokenMs: firstTokenMs}, fmt.Errorf("stream data interval timeout")
+
+		case <-keepaliveCh:
+			if cw.Disconnected() {
+				continue
+			}
+			if time.Since(lastDataAt) < keepaliveInterval {
+				continue
+			}
+			// SSE ping 事件：Anthropic 原生格式，客户端会正确处理，
+			// 同时保持连接活跃防止 Cloudflare Tunnel 等代理断开
+			if !cw.Fprintf("event: ping\ndata: {\"type\": \"ping\"}\n\n") {
+				logger.LegacyPrintf("service.antigravity_gateway", "Client disconnected during keepalive ping (antigravity claude), continuing to drain upstream for billing")
+				continue
+			}
 		}
 	}
 }
@@ -4299,6 +4362,22 @@ func (s *AntigravityGatewayService) streamUpstreamResponse(c *gin.Context, resp
 		intervalCh = intervalTicker.C
 	}
 
+	// 下游 keepalive：防止代理/Cloudflare Tunnel 因连接空闲而断开
+	keepaliveInterval := time.Duration(0)
+	if s.settingService.cfg != nil && s.settingService.cfg.Gateway.StreamKeepaliveInterval > 0 {
+		keepaliveInterval = time.Duration(s.settingService.cfg.Gateway.StreamKeepaliveInterval) * time.Second
+	}
+	var keepaliveTicker *time.Ticker
+	if keepaliveInterval > 0 {
+		keepaliveTicker = time.NewTicker(keepaliveInterval)
+		defer keepaliveTicker.Stop()
+	}
+	var keepaliveCh <-chan time.Time
+	if keepaliveTicker != nil {
+		keepaliveCh = keepaliveTicker.C
+	}
+	lastDataAt := time.Now()
+
 	flusher, _ := c.Writer.(http.Flusher)
 	cw := newAntigravityClientWriter(c.Writer, flusher, "antigravity upstream")
 
@@ -4316,6 +4395,8 @@ func (s *AntigravityGatewayService) streamUpstreamResponse(c *gin.Context, resp
 				return &antigravityStreamResult{usage: usage, firstTokenMs: firstTokenMs}
 			}
 
+			lastDataAt = time.Now()
+
 			line := ev.line
 
 			// 记录首 token 时间
@@ -4341,6 +4422,20 @@ func (s *AntigravityGatewayService) streamUpstreamResponse(c *gin.Context, resp
 			}
 			logger.LegacyPrintf("service.antigravity_gateway", "Stream data interval timeout (antigravity upstream)")
 			return &antigravityStreamResult{usage: usage, firstTokenMs: firstTokenMs}
+
+		case <-keepaliveCh:
+			if cw.Disconnected() {
+				continue
+			}
+			if time.Since(lastDataAt) < keepaliveInterval {
+				continue
+			}
+			// SSE ping 事件：Anthropic 原生格式，客户端会正确处理，
+			// 同时保持连接活跃防止 Cloudflare Tunnel 等代理断开
+			if !cw.Fprintf("event: ping\ndata: {\"type\": \"ping\"}\n\n") {
+				logger.LegacyPrintf("service.antigravity_gateway", "Client disconnected during keepalive ping (antigravity upstream), continuing to drain upstream for billing")
+				continue
+			}
 		}
 	}
 }