feat: migrate web search tool to support Tavily alongside DuckDuckGo

[tavily-integrations]

Summary

Adds Tavily as a configurable primary web search provider alongside the existing DuckDuckGo integration in WebSearchTool. When TAVILY_API_KEY is set, Tavily is used as the primary search path; DDGS serves as the automatic fallback when the key is absent or on Tavily error. This is an additive/non-breaking change.

Files Changed

• tools/mcp/engine.py — Refactored execute_impl() into _tavily_search() and _ddgs_search() methods. Tavily is tried first when API key is configured; DDGS is the fallback.
• core/config.py — Added tavily_api_key: str = '' field to Settings class.
• pyproject.toml — Added tavily-python>=0.5.0 to project dependencies.
• .env.example — Documented TAVILY_API_KEY under LLM Provider API Keys section.

Dependency Changes

• Added tavily-python>=0.5.0 to [project].dependencies in pyproject.toml

Environment Variable Changes

• Added TAVILY_API_KEY (optional) — when set, enables Tavily as primary web search provider

Notes for Reviewers

• DuckDuckGo remains the zero-config default; no existing behavior changes unless TAVILY_API_KEY is provided.
• Tavily response fields (content, url) differ from DDGS (body, href); both are handled in their respective methods.
• Caching behavior is unchanged — the shared execute() wrapper applies to both paths.

Automated Review

• Passed after 1 attempt(s)
• Final review: The Tavily migration is correct, complete, and non-regressive. The implementation properly adds Tavily as the primary search provider with automatic DDGS fallback, uses the correct tavily-python SDK patterns (TavilyClient.search()), wraps the synchronous client in asyncio.to_thread, maps the correct Tavily response fields (title, content, url), and keeps the DDGS path intact. All four reported files are updated consistently: dependency added to pyproject.toml, config field added to core/config.py, env var documented in .env.example, and logic added to tools/mcp/engine.py. Two minor issues noted below but neither blocks approval.