fix(langchain): make RunnableRails.transform/atransform proper (async) iterators

[nac7]

Summary

Fixes #1692.

RunnableRails.transform and RunnableRails.atransform were aliased to invoke/ainvoke, which violates the LangChain Runnable streaming protocol:

• transform must consume an iterator of inputs and return an iterator of outputs.
• atransform must consume an async iterator of inputs and must itself be an async iterator.

Because atransform was an async def that returned a value, calling it produced a coroutine rather than an async iterator. As a result, placing RunnableRails inside a RunnableSequence and streaming it raised:

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

Reproduction (from the issue)

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

async def reproduce_bug():
    config = RailsConfig.from_content(yaml_content="models: []")
    rails = RunnableRails(config, passthrough=True)
    pipeline = RunnableLambda(lambda x: x) | rails
    async for _ in pipeline.astream("hello"):  # TypeError before this fix
        pass

asyncio.run(reproduce_bug())

Fix

Defer to the base Runnable.transform/atransform implementations, which buffer the input stream and delegate to the already-correct stream/astream methods. Guardrails therefore run once over the fully assembled input, with no per-item re-invocation.

Note: this intentionally avoids the per-item invoke/ainvoke approach that led the earlier attempts (#1702, #1763) to be closed — stream/astream already handle a single assembled input correctly.

Tests

Added regression tests in tests/integrations/langchain/runnable_rails/test_streaming.py:

• RunnableRails nested in a RunnableSequence is streamable via astream (the exact issue scenario) and stream.
• atransform returns an async iterator (not a coroutine).
• transform consumes an iterator and yields outputs.

All four fail on the previous code and pass with this fix. Full runnable_rails suite: 165 passed, 2 skipped.