fix: wrap blocking readability call with asyncio.to_thread in web_fetch (#2157)

* fix: wrap blocking readability call with asyncio.to_thread in web_fetch

The readability extractor internally spawns a Node.js subprocess via
readabilipy, which blocks the async event loop and causes a
BlockingError when web_fetch is invoked inside LangGraph's async
runtime.

Wrap the synchronous extract_article call with asyncio.to_thread to
offload it to a thread pool, unblocking the event loop.

Note: community/infoquest/tools.py has the same latent issue and
should be addressed in a follow-up PR.

Closes #2152

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* test: verify web_fetch offloads extraction via asyncio.to_thread

Add a regression test that monkeypatches asyncio.to_thread to confirm
readability extraction is offloaded to a worker thread, preventing
future refactors from reintroducing the blocking call.

Addresses Copilot review feedback on #2157.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/packages/harness/deerflow/community/jina_ai/tools.py

@@ -1,3 +1,5 @@
+import asyncio
+
 from langchain.tools import tool
 
 from deerflow.community.jina_ai.jina_client import JinaClient
@@ -26,5 +28,5 @@ async def web_fetch_tool(url: str) -> str:
     html_content = await jina_client.crawl(url, return_format="html", timeout=timeout)
     if isinstance(html_content, str) and html_content.startswith("Error:"):
         return html_content
-    article = readability_extractor.extract_article(html_content)
+    article = await asyncio.to_thread(readability_extractor.extract_article, html_content)
     return article.to_markdown()[:4096]