Fix Langfuse callback passthrough in RunnableRails (#472)

[divya-garak]

Summary

Fixes #472: This PR resolves the issue where Langfuse tracing integration was broken when using NeMo-Guardrails with LangChain chains.

The problem was that the RunnableConfig containing callbacks wasn't being properly passed through to the underlying LLM runnable in the passthrough function.

Changes Made

• Store config and kwargs: Modified RunnableRails.invoke() to store the RunnableConfig and kwargs for later use in the passthrough function
• Pass config to underlying runnable: Updated _init_passthrough_fn() to use the stored config when invoking the passthrough runnable
• Added comprehensive test: Created test_runnable_config_callback_passthrough() to verify that callbacks are properly propagated

Technical Details

The issue occurred because:

1. RunnableRails.invoke() received a config parameter with callbacks
2. The passthrough_fn was called asynchronously through the Rails system
3. The original config wasn't available in the passthrough function scope
4. Callbacks (like Langfuse tracing) weren't being called

Testing

• ✅ All existing tests continue to pass
• ✅ New test verifies callback passthrough functionality
• ✅ Tested with passthrough mode scenarios

This ensures that tracing libraries like Langfuse work correctly when using RunnableRails in LangChain chains.

[Copilot]

Pull Request Overview

This PR fixes Langfuse callback passthrough in RunnableRails when integrated with LangChain chains by ensuring the RunnableConfig containing callbacks is properly propagated to the underlying runnable.

• Modified RunnableRails to store config and kwargs from invoke() method for later use in passthrough function
• Updated passthrough function to use stored config when invoking the underlying runnable
• Added comprehensive test to verify callback propagation functionality

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
nemoguardrails/integrations/langchain/runnable_rails.py	Added instance variables to store current config/kwargs and modified passthrough function to use them
tests/test_runnable_rails.py	Added test case to verify RunnableConfig callbacks are properly passed through to underlying runnable

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[Copilot]

There's duplicate assignment here. Both self.config and self._current_config are assigned the same value, as are self.kwargs and self._current_kwargs. Consider removing the redundant assignments to self.config and self.kwargs since they appear to be legacy variables that are now replaced by the _current_* variants.

Suggested change

	self.config = config
	self.kwargs = kwargs