* feat(community): migrate tavily tools to AsyncTavilyClient
Replace the synchronous TavilyClient with AsyncTavilyClient in the tavily
community tools, as tracked in backend/docs/TODO.md planned features
('Replace sync requests with httpx.AsyncClient in community tools').
web_search_tool and web_fetch_tool become async functions and await
client.search()/client.extract(). External behavior (return values,
error strings, credentials handling) is unchanged; the client is still
constructed with api_key=None when the tool config has no key, letting
the SDK read TAVILY_API_KEY from the environment.
Tests are updated to invoke the async tools via ainvoke under
@pytest.mark.anyio, and the credential assertions now verify the client
is constructed with the right api_key (AsyncTavilyClient keeps the key
inside its httpx client and exposes no public api_key attribute).
* fix(tavily): close async clients after each tool request
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
