feat: add optional Tavily search provider

[wd041216-bit]

Summary

• Re-submit the unmerged Tavily provider work against the current zero_api_key_web_search package layout
• Add an optional TavilyProvider for text/news searches, enabled by ZERO_SEARCH_TAVILY_API_KEY or TAVILY_API_KEY
• Add the tavily optional dependency extra and provider discovery/status wiring
• Cover provider activation, explicit configuration errors, result normalization, and unsupported search types in tests

Replaces

• Supersedes the closed, unmerged Tavily PRs feat: add Tavily as parallel search engine alongside DuckDuckGo #2, feat: add Tavily as optional parallel search engine alongside DuckDuckGo #3, feat: expose Tavily availability in MCP server tool description #4, and feat: add Tavily as optional search provider alongside ddgs and SearXNG #6, whose diffs targeted older package paths or external bot-authored commits.

Validation

• ruff check zero_api_key_web_search/ tests/
• pyright zero_api_key_web_search/
• coverage run -m unittest discover -s tests -v && coverage report --fail-under=80 --include="zero_api_key_web_search/*"
• python benchmarks/run_benchmark.py --json
• python -m build
• twine check dist/*

Identity

• Commit authored and committed as wd041216-bit wd041216@uw.edu.