fix: support async LangChain tools in convert_tool()

[Ricardo-M-L]

Summary

Fixes #1402

• Problem: LangChainInteroperability.convert_tool() always wraps LangChain tools with a synchronous function that calls langchain_tool.run(), even when the tool provides native async support via _arun. This blocks the event loop when used inside a_initiate_chat() and other async execution paths.
• Solution: Added async detection via MRO inspection. When a LangChain tool overrides _arun (i.e., provides a real async implementation beyond BaseTool's default NotImplementedError stub), the converted tool now uses an async def function that calls ainvoke(). Tools without async support continue to use the synchronous run() method, preserving full backward compatibility.
• How it works with the framework: AG2's a_execute_function() already checks is_coroutine_callable(func) and properly awaits async functions. By providing an async function when the LangChain tool supports it, the tool integrates naturally with the existing async execution path.

Changes

autogen/interop/langchain/langchain_tool.py:

• Added _langchain_tool_has_async_implementation() helper that walks the MRO to detect if the tool's _arun is overridden from BaseTool's default
• Modified convert_tool() to create an async wrapper using ainvoke() when async support is detected
• Sync path remains unchanged for backward compatibility

test/interop/langchain/test_langchain_async.py (new):

• Tests that sync-only tools remain sync after conversion
• Tests that async tools produce async converted functions
• Tests async tool execution via await
• Tests concurrent execution (async tool doesn't block event loop)
• Tests metadata preservation, error handling
• Tests for the _langchain_tool_has_async_implementation detection helper

Test plan

• pytest test/interop/langchain/test_langchain_async.py — new async tests pass
• pytest test/interop/langchain/test_langchain.py — existing sync tests still pass (regression)
• Verify async detection correctly identifies tools with/without _arun override

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 6.66667% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
autogen/interop/langchain/langchain_tool.py	6.66%	14 Missing ⚠️

Files with missing lines	Coverage Δ
autogen/interop/langchain/langchain_tool.py	44.44% <6.66%> (-18.89%)	⬇️

... and 122 files with indirect coverage changes

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