dsh-web-search-order: Ordered Graceful Degradation Plan for DSH Search Providers

This plugin adds ordered fallback logic to DSH's `web_search` tool. By configuring provider order, timeouts, and an exclusion list, it automatically switches between registered search providers (such as exa and deepseek-official). When a provider is unavailable, times out, or returns empty results, the plugin tries the next one until it succeeds or all providers fail. It supports hot-reloadable configuration and is compatible with multiple DSH versions.