Multi-engine web search with full parameter control. Supports Tavily, Exa, Serper, and SerpAPI with domain filtering, date ranges, deep search, news mode, and content extraction. Auto-selects the best engine based on query type and available API keys. 多引擎精细化搜索：支持域名过滤、日期范围、深度搜索、新闻模式、内容提取。 根据查询类型和可用 API Key 自动选择最优引擎。