dsh-search-failover: DSH's provider-level search and crawling pool

dsh-search-failover is a DSH plugin maintained by Walvez, designed to address issues such as single backend quota exhaustion or network failures. It transparently takes over DSH's `web_search` and `web_fetch` functions at the Provider level, preserving native tool signatures while integrating multiple search and fetch backends into a unified scheduling pool. The plugin's core functions include: support for dual routing strategies of "priority-based Failover" and "weighted round-robin"; intelligent quota awareness and circuit breaker mechanisms that apply different cooling periods for quota exhaustion versus transient errors, along with automatic health checks; provision of the `web_search_from` skill to allow the AI to autonomously select engines like Exa and Tavily as alternative sources; and support for rotating multiple API keys within the same backend to maximize quota utilization.