🤖 feat: Add Volcengine Web Search Provider

[nothingp]

Summary

Add Volcengine (火山引擎) as a new web search provider alongside Serper, SearXNG, and Tavily.

Changes

• volcengine-search.ts — New provider supporting both web (JSON response) and web_summary (SSE stream) search
  types via POST https://open.feedcoopapi.com/search_api/web_search
• volcengine.test.ts — 12 unit tests covering SSE/JSON parsing, payload construction, error handling, and edge
  cases
• types.ts — Added 'volcengine' to SearchProvider type and volcengine config fields
• search.ts / tool.ts — Wire volcengine dispatch and config into createSearchAPI / createSearchTool

Auth

Bearer token via VOLCENGINE_API_KEY environment variable or volcengineApiKey config field.