feat: add support for system messages to RunnableRails

[smruthi33]

Description

The current implementation does not support messages that have a System Message

guardrails_config = RailsConfig.from_path(<Path>)
guardrails = RunnableRails(guardrails_config)
client = ChatOpenAI(
                base_url=GEN_AI_ENDPOINT,
                api_key=GEN_AI_ENDPOINT_AUTHORIZATION,
                model= MODEL_NAME,
                temperature=0                
            )
messages = [
                (
                    "system", system
                )
                ,(
                    "human", prompt
                )
            ]
prompt = ChatPromptTemplate.from_messages(messages)

chain_with_guardrails =  prompt | (guardrails | client)

In this setup, RunnableRails does not handle messages containing system roles. Attempting to pass such a structure as a dictionary causes conflicts with the format expected by the Runnable interface from LangChain, which assumes a flat input structure and does not support multiple message types out of the box. This change addresses that limitation.

Related Issue(s)

Checklist

• I've read the CONTRIBUTING guidelines.
• I've updated the documentation if applicable.
• I've added tests if applicable.
• @mentions of the person or team responsible for reviewing proposed changes.

[Pouyanpi]

Thank you @smruthi33 for opening this PR 🚀

Would be great if you can add a test to test_runnable_rails.py. Thank you!