fix: async from_litellm

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:rocket:	This description was created by Ellipsis for commit 12a8fbc47550d892de49fef598d8e65c38b446cf

Fix from_litellm to correctly detect async functions and add corresponding tests.

Key points:

Fix from_litellm function in instructor/client.py to correctly detect async functions using inspect.iscoroutinefunction.
Add tests in tests/llm/test_litellm.py to verify Instructor and AsyncInstructor creation.
Ensure from_litellm correctly returns Instructor for sync functions and AsyncInstructor for async functions.

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jxnl / instructor