feat: add Tavily as configurable search provider in search_web.py

[tavily-integrations]

Summary

Adds Tavily Search as a configurable alternative to Serper in the web search tool, controlled by the SEARCH_PROVIDER environment variable.

• Default behavior unchanged: SEARCH_PROVIDER=serper (the default) routes all queries through the existing Serper path
• Tavily option: Setting SEARCH_PROVIDER=tavily routes queries through the new tavily_search() function using tavily-python SDK
• Output format consistency: Tavily results are formatted in the same numbered markdown style as Serper results for downstream compatibility
• Batched query support preserved: Both single-string and array-of-queries inputs work with either provider

Files Changed

• tools/tool/search_web.py — Added TavilyClient import, SEARCH_PROVIDER env var, tavily_search() function, and _search_single() dispatcher
• requirements.txt — Added tavily-python dependency
• config/.env — Added TAVILY_API_KEY and SEARCH_PROVIDER=serper placeholders

Dependency Changes

• Added tavily-python to requirements.txt

Environment Variable Changes

• Added TAVILY_API_KEY — API key for Tavily search (required when SEARCH_PROVIDER=tavily)
• Added SEARCH_PROVIDER — Controls which search backend to use (serper or tavily, defaults to serper)

Notes for Reviewers

• All existing Serper logic is untouched; this is a purely additive change
• The tavily_search function uses search_depth="basic" and max_results=10 to match the Serper behavior
• Error handling in tavily_search mirrors the Serper error patterns

Automated Review

• Passed after 1 attempt(s)
• Final review: The migration correctly adds Tavily as a configurable search provider alongside Serper. The implementation is additive, defaults remain unchanged (serper), dependencies and env vars are documented, and the dispatcher pattern is clean. A few minor issues exist but none are blocking.