fix(agents): auto-await coroutines in async middleware

[majiayu000]

Summary

• Fixes AttributeError: 'coroutine' object has no attribute 'result' when using langchain.agents.create_agent with async execution (ainvoke) and custom async middleware #34234
• Adds _normalize_to_model_response_async() to detect when async middleware returns a coroutine instead of ModelResponse/AIMessage
• Auto-awaits the coroutine with a helpful warning message guiding users to fix their code

Problem

When users implement awrap_model_call and forget to await the handler:

async def awrap_model_call(self, request, handler):
    return handler(request)  # Missing await!

This causes AttributeError: 'coroutine' object has no attribute 'result' which is confusing.

Solution

The fix detects this scenario and:

1. Auto-awaits the coroutine to prevent the crash
2. Emits a warning explaining the issue and how to fix it

Test plan

• Added tests for missing await scenarios (single/multiple middleware)
• Added test verifying proper await does not emit warning
• All existing middleware tests pass (167 tests)
• Linting passes
• Type checking passes

AI agents were involved in developing this contribution.

[hankbesser]

Sorry if this comes off harsh, But why add 147 lines to an already loaded factory.py when this entails giving a warning when not adding the correct 'await' before the asynchronous handler. Also technically, if downstream middleware short circuits the system, you might not want to await the handler and have the bare coroutine move down stream.

I would say this is more of a documentation point as there isn't really any asynchronous specific documentation for middleware anywhere. But those are just my two cents. Thanks for the work though.