web_search_tool now tries Bing Web Search API first (works in China, free tier
1000 searches/month via Azure), then falls back to DuckDuckGo. Both backends
support optional SEARCH_PROXY for network-restricted environments.
New settings:
- BING_SEARCH_API_KEY: Azure Bing Search API key
- SEARCH_PROXY: HTTP proxy for outbound search requests
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>