fix(antigravity): Gemini 原生流对 go-genai / python-genai 客户端不发 SSE 注释心跳

Antigravity 账号的 Gemini 原生流式路径在上游空闲超过 gateway.stream_keepalive_interval
（默认 10s）时会向下游写 ":\n\n" 作为心跳。SSE 规范允许注释行，但 Google 官方 SDK 里
go-genai 与 python-genai 都不会忽略它：

- go-genai（Antigravity CLI 在用）api_client.go iterateResponseStream 对任何非 data: 前缀的
  事件直接返回 "iterateResponseStream: invalid stream chunk: :"，整个流中断；
- python-genai _api_client.py 把非 data: 行当作错误 JSON 拼接后 json.loads，同样抛错。

Google 官方 API 从不下发注释行，这两个 SDK 从未处理过这种输入。js-genai 用规范的
event-stream 解析器，不受影响。

现在按下游 User-Agent / X-Goog-Api-Client 识别这两类 SDK（形如
google-genai-sdk/1.71.0 gl-go/go1.28 …、google-genai-sdk/x gl-python/3.12），命中时该路径
不再发注释心跳；其他客户端（curl、js-genai、Gemini CLI）行为不变。

生产复现：Antigravity CLI 1.2.5 经网关（sub2api 0.2.5）跑长思考请求，上游首个 chunk 超过 10s
时客户端报 "agent executor error: … iterateResponseStream: invalid stream chunk: :"。

backend/internal/service/antigravity_gateway_streaming.go

@@ -197,6 +197,11 @@ func (s *AntigravityGatewayService) handleGeminiStreamingResponse(c *gin.Context
 	if s.settingService.cfg != nil && s.settingService.cfg.Gateway.StreamKeepaliveInterval > 0 {
 		keepaliveInterval = time.Duration(s.settingService.cfg.Gateway.StreamKeepaliveInterval) * time.Second
 	}
+	// go-genai / python-genai 不会忽略 SSE 注释行，收到 ":\n\n" 会直接把整个流判成
+	// invalid stream chunk 而中断（Antigravity CLI 在用 go-genai）。对这类客户端宁可不发心跳。
+	if keepaliveInterval > 0 && downstreamRejectsSSEComments(c) {
+		keepaliveInterval = 0
+	}
 	var keepaliveTicker *time.Ticker
 	if keepaliveInterval > 0 {
 		keepaliveTicker = time.NewTicker(keepaliveInterval)

backend/internal/service/gemini_sse_comment_compat.go

@@ -0,0 +1,35 @@
+package service
+
+import (
+	"strings"
+
+	"github.com/gin-gonic/gin"
+)
+
+// geminiClientRejectsSSEComments 判断下游客户端是否是「无法容忍 SSE 注释行」的 Google GenAI SDK。
+//
+// Gemini 原生流里我们用 ":\n\n" 作为空闲心跳（防止代理/Cloudflare Tunnel 断开），但官方 SDK 里
+// 有两个实现不按 SSE 规范忽略注释行：
+//   - go-genai（Antigravity CLI 在用）：api_client.go iterateResponseStream 对任何非 "data:" 前缀的
+//     事件直接报 "invalid stream chunk: :"，整个流中断；
+//   - python-genai：_api_client.py 把非 "data:" 行当作错误 JSON 拼接后 json.loads，同样抛错。
+//
+// js-genai 用的是规范的 event-stream 解析器，能正确忽略注释。Google 官方 API 从不下发注释行，
+// 所以这两个 SDK 从未处理过这种输入。识别依据是 SDK 固定携带的 User-Agent / X-Goog-Api-Client，
+// 形如 "google-genai-sdk/1.71.0 gl-go/go1.28 ..." 或 "google-genai-sdk/1.x gl-python/3.12"。
+func geminiClientRejectsSSEComments(clientHint string) bool {
+	hint := strings.ToLower(strings.TrimSpace(clientHint))
+	if !strings.Contains(hint, "google-genai-sdk/") {
+		return false
+	}
+	return strings.Contains(hint, "gl-go/") || strings.Contains(hint, "gl-python/")
+}
+
+// downstreamRejectsSSEComments 从请求头识别下游客户端（同时看 User-Agent 与 X-Goog-Api-Client）。
+func downstreamRejectsSSEComments(c *gin.Context) bool {
+	if c == nil || c.Request == nil {
+		return false
+	}
+	return geminiClientRejectsSSEComments(c.GetHeader("User-Agent")) ||
+		geminiClientRejectsSSEComments(c.GetHeader("X-Goog-Api-Client"))
+}

backend/internal/service/gemini_sse_comment_compat_test.go

@@ -0,0 +1,95 @@
+package service
+
+import (
+	"io"
+	"net/http"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGeminiClientRejectsSSEComments(t *testing.T) {
+	cases := []struct {
+		name string
+		hint string
+		want bool
+	}{
+		{"go-genai (Antigravity CLI)", "google-genai-sdk/1.71.0 gl-go/go1.28-20260721-RC03 cl/951519500 +3ebc191975 X:fieldtrack,boringcrypto", true},
+		{"python-genai", "google-genai-sdk/1.20.0 gl-python/3.12.4", true},
+		{"js-genai tolerates comments", "google-genai-sdk/1.9.0 gl-node/22.3.0", false},
+		{"gemini-cli", "GeminiCLI/0.60.0 (darwin; arm64)", false},
+		{"curl", "curl/8.7.1", false},
+		{"empty", "", false},
+		{"case-insensitive", "Google-GenAI-SDK/1.0.0 GL-Go/go1.27", true},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			require.Equal(t, tc.want, geminiClientRejectsSSEComments(tc.hint))
+		})
+	}
+}
+
+func TestDownstreamRejectsSSECommentsReadsBothHeaders(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	c, _ := newAntigravityCompatContext(http.MethodPost, "/v1beta/models/gemini-3.8-flash:streamGenerateContent", nil)
+	require.False(t, downstreamRejectsSSEComments(c))
+
+	c.Request.Header.Set("X-Goog-Api-Client", "google-genai-sdk/1.71.0 gl-go/go1.28")
+	require.True(t, downstreamRejectsSSEComments(c))
+
+	c.Request.Header.Del("X-Goog-Api-Client")
+	c.Request.Header.Set("User-Agent", "google-genai-sdk/1.71.0 gl-go/go1.28")
+	require.True(t, downstreamRejectsSSEComments(c))
+
+	require.False(t, downstreamRejectsSSEComments(nil))
+}
+
+// runAntigravityGeminiStreamWithIdle 起一条上游流：先发一个 data 事件，然后空闲 idle 时长再关闭，
+// 返回写给下游的全部字节。用来观察空闲期间网关是否发了 ":\n\n" 心跳。
+func runAntigravityGeminiStreamWithIdle(t *testing.T, userAgent string, idle time.Duration) string {
+	t.Helper()
+	gin.SetMode(gin.TestMode)
+	svc := newAntigravityCompatService(
+		config.GatewayConfig{MaxLineSize: defaultMaxLineSize, StreamKeepaliveInterval: 1},
+		nil,
+	)
+	c, recorder := newAntigravityCompatContext(http.MethodPost, "/v1beta/models/gemini-3.8-flash:streamGenerateContent", nil)
+	if userAgent != "" {
+		c.Request.Header.Set("User-Agent", userAgent)
+	}
+	reader, writer := io.Pipe()
+	resp := &http.Response{StatusCode: http.StatusOK, Header: http.Header{}, Body: reader}
+	done := make(chan error, 1)
+	go func() {
+		_, err := svc.handleGeminiStreamingResponse(c, resp, time.Now())
+		done <- err
+	}()
+	_, err := io.WriteString(
+		writer,
+		`data: {"response":{"responseId":"resp_1","candidates":[{"content":{"parts":[{"text":"partial"}]}}],"usageMetadata":{"promptTokenCount":8,"candidatesTokenCount":1}}}`+"\n\n",
+	)
+	require.NoError(t, err)
+	time.Sleep(idle)
+	require.NoError(t, writer.Close())
+	require.NoError(t, <-done)
+	require.NoError(t, reader.Close())
+	return recorder.Body.String()
+}
+
+func TestAntigravityGeminiStreamKeepsCommentKeepaliveForOrdinaryClients(t *testing.T) {
+	out := runAntigravityGeminiStreamWithIdle(t, "curl/8.7.1", 1200*time.Millisecond)
+	require.Contains(t, out, ":\n\n", "ordinary clients should still get the idle keepalive")
+	require.Contains(t, out, `"text":"partial"`)
+}
+
+func TestAntigravityGeminiStreamSkipsCommentKeepaliveForGoGenai(t *testing.T) {
+	out := runAntigravityGeminiStreamWithIdle(t, "google-genai-sdk/1.71.0 gl-go/go1.28-20260721-RC03", 1200*time.Millisecond)
+	require.Contains(t, out, `"text":"partial"`)
+	for _, event := range strings.Split(out, "\n\n") {
+		require.False(t, strings.HasPrefix(event, ":"), "go-genai must never receive an SSE comment event, got %q", event)
+	}
+}