fix: make error passthrough effective for non-failover upstream errors

backend/internal/service/gemini_messages_compat_service.go

@@ -1498,6 +1498,28 @@ func (s *GeminiMessagesCompatService) writeGeminiMappedError(c *gin.Context, acc
 		log.Printf("[Gemini] upstream error %d: %s", upstreamStatus, truncateForLog(body, s.cfg.Gateway.LogUpstreamErrorBodyMaxBytes))
 	}
 
+	if status, errType, errMsg, matched := applyErrorPassthroughRule(
+		c,
+		PlatformGemini,
+		upstreamStatus,
+		body,
+		http.StatusBadGateway,
+		"upstream_error",
+		"Upstream request failed",
+	); matched {
+		c.JSON(status, gin.H{
+			"type":  "error",
+			"error": gin.H{"type": errType, "message": errMsg},
+		})
+		if upstreamMsg == "" {
+			upstreamMsg = errMsg
+		}
+		if upstreamMsg == "" {
+			return fmt.Errorf("upstream error: %d (passthrough rule matched)", upstreamStatus)
+		}
+		return fmt.Errorf("upstream error: %d (passthrough rule matched) message=%s", upstreamStatus, upstreamMsg)
+	}
+
 	var statusCode int
 	var errType, errMsg string