[优化] 更新 openresty_observability_port 描述，增强健康检查逻辑，使用 stub_status 代替 openresty -t

openflare_agent/internal/nginx/manager.go

@@ -788,6 +788,7 @@ func (m *Manager) checkStubStatus(ctx context.Context) error {
 	if resp.StatusCode != http.StatusOK {
 		return fmt.Errorf("openresty health endpoint returned %s", resp.Status)
 	}
+	slog.Debug("openresty health endpoint is healthy", "url", openrestyStubUrl)
 	return nil
 }