fix(antigravity): 扩展 isSignatureRelatedError 检测 thinking 结构错误

- 添加对 "Expected thinking/redacted_thinking" 错误的检测
- 修复 antigravity 服务中 thinking 模式启用时的结构约束错误
- 确保此类错误能触发重试逻辑

backend/internal/service/antigravity_gateway_service.go

@@ -574,7 +574,17 @@ func isSignatureRelatedError(respBody []byte) bool {
 	}
 
 	// Keep this intentionally broad: different upstreams may use "signature" or "thought_signature".
-	return strings.Contains(msg, "thought_signature") || strings.Contains(msg, "signature")
+	if strings.Contains(msg, "thought_signature") || strings.Contains(msg, "signature") {
+		return true
+	}
+
+	// Also detect thinking block structural errors:
+	// "Expected `thinking` or `redacted_thinking`, but found `text`"
+	if strings.Contains(msg, "expected") && (strings.Contains(msg, "thinking") || strings.Contains(msg, "redacted_thinking")) {
+		return true
+	}
+
+	return false
 }
 
 func extractAntigravityErrorMessage(body []byte) string {