feat(integration): add GuardrailsMiddleware for LangChain agent

[Pouyanpi]

#‪# Description

Add a new GuardrailsMiddleware class for seamless integration with LangChain's Agent Middleware system. The middleware intercepts agent conversations to apply NeMo Guardrails input/output validation before and after model calls.

This follows the LangChain Middleware pattern introduced in LangChain 1.0, providing before_model and after_model hooks for precise control over the agent loop.

Key features:

• GuardrailsMiddleware - Full middleware with both input and output rails
• InputRailsMiddleware - Input-only validation
• OutputRailsMiddleware - Output-only validation
• GuardrailViolation exception for programmatic error handling
• Configurable blocking behavior (raise exception vs return blocked message)
• Support for both config_path and config_yaml initialization

Usage example:

from langchain.agents import create_agent
from nemoguardrails.integrations.langchain.middleware import GuardrailsMiddleware

guardrails = GuardrailsMiddleware(
    config_path="./config",
    raise_on_violation=False,
)

agent = create_agent("openai:gpt-4o", middleware=[guardrails])

References

• LangChain Middleware Overview
• LangChain Middleware Reference
• LangChain Agent Middleware Blog Post

[codecov]

Codecov Report

❌ Patch coverage is 89.32039% with 22 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...emoguardrails/integrations/langchain/middleware.py	84.09%	21 Missing ⚠️
nemoguardrails/rails/llm/llmrails.py	98.21%	1 Missing ⚠️

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