bug: RunnableRails.atransform is not returning an async interator

[sfitts]

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Python version (python --version)

Python 3.12.9

Operating system/version

Windows 11

NeMo-Guardrails version (if you must use a specific version and not the latest

0.20.0

Describe the bug

When using RunnableRails in any chain that uses a RunnableSequence we see the following error:

TypeError: 'async for' requires an object with __aiter__ method, got coroutine

The issue seems to have been introduced with version 0.17.0. I've attached a reproduction involving just LC and NemoRails (full disclosure, it was generated by Claude Code, though I've edited it a bit for brevity and I am a person ;)).

Steps To Reproduce

import asyncio
import inspect

from langchain_core.runnables import RunnableLambda
from nemoguardrails import RailsConfig
from nemoguardrails.integrations.langchain.runnable_rails import RunnableRails

async def reproduce_bug():
    print("=== Bug reproduction ===")
    config = RailsConfig.from_content(yaml_content="models: []")
    rails = RunnableRails(config, passthrough=True)

    # Putting RunnableRails in a sequence triggers atransform via astream
    pipeline = RunnableLambda(lambda x: x) | rails

    try:
        async for _ in pipeline.astream("hello"):
            pass
        print("No error — bug not reproduced (unexpected)")
    except TypeError as e:
        print(f"TypeError (expected): {e}")
    print()


if __name__ == "__main__":
    asyncio.run(reproduce_bug())

Expected Behavior

When run, the above code should output "No error -- bug not reproduced".

Actual Behavior

The current output is:

$ python  nemo_atransform_repro.py
=== Bug reproduction ===
No main LLM specified in the config and no LLM provided via constructor.
TypeError (expected): 'async for' requires an object with __aiter__ method, got coroutine
<path elided>/nemo_atransform_repro.py:21: RuntimeWarning: coroutine 'RunnableRails.atransform' was never awaited
  print(f"TypeError (expected): {e}")
RuntimeWarning: Enable tracemalloc to get the object allocation traceback