feat: add Tavily as a configurable web search provider option

[tavily-integrations]

Summary

• Added Tavily as a fourth selectable web search engine provider in the PyGPT web search plugin
• Created TavilySearch provider class following the existing BaseProvider pattern (mirroring DuckDuckGo/Google/Bing providers)
• Registered the new provider in app.py alongside existing providers
• Added tavily-python dependency to pyproject.toml

Files changed

• src/pygpt_net/provider/web/tavily_search.py — New file: TavilySearch provider with search(), is_configured(), init_options(), and get_config_message() methods
• src/pygpt_net/app.py — Added import and registration of TavilySearch
• pyproject.toml — Added tavily-python dependency

Dependency changes

• Added tavily-python = "^0.5.0" to [tool.poetry.dependencies]

Environment variable changes

• TAVILY_API_KEY — Users configure this via the tavily_api_key plugin setting in the Tavily tab

Notes for reviewers

• This is an additive change — all existing providers are untouched
• The user selects the active provider via the existing plugin settings UI dropdown; no routing logic changes were needed
• The Tavily provider uses search_depth="basic" (1 credit per search); "advanced" (2 credits) can be exposed as a setting later if desired
• Tavily API supports up to 20 results per query (vs 10 for Google/DDG)

🤖 Generated with Claude Code

Automated Review

• Passed after 1 attempt(s)
• Final review: The Tavily provider implementation is correct, well-scoped, and consistent with the existing codebase patterns. All three reported files are properly modified: the new TavilySearch class follows the BaseProvider contract, the app.py registration mirrors existing providers, and tavily-python is added to pyproject.toml. Tavily SDK usage is correct (client initialization, search() call, response parsing via response["results"] and result["url"]). The lazy-import _load_tavily() pattern is consistent with the DuckDuckGo provider. Only minor issues found — no blockers.