fix: update antigravity user-agent version to 1.19.6

Update the default user-agent version from 1.18.4 to 1.19.6
to match the latest official antigravity client.

backend/internal/pkg/antigravity/oauth_test.go

@@ -690,7 +690,7 @@ func TestConstants_值正确(t *testing.T) {
 	if RedirectURI != "http://localhost:8085/callback" {
 		t.Errorf("RedirectURI 不匹配: got %s", RedirectURI)
 	}
-	if GetUserAgent() != "antigravity/1.18.4 windows/amd64" {
+	if GetUserAgent() != "antigravity/1.19.6 windows/amd64" {
 		t.Errorf("UserAgent 不匹配: got %s", GetUserAgent())
 	}
 	if SessionTTL != 30*time.Minute {