feat: parsing third-party messages for mellea

[psschwei]

Misc PR

Type of PR

• Bug Fix
• New Feature
• Documentation
• Other

Description

• Link to Issue: add support for ingesting chat contexts / chat completion messages from other libraries #255

• Adds support for converting LangChain messages to Mellea Message types and validating LangChain agent outputs using Mellea's native validation API
• Adds a high-level API for validating outputs from external LLM frameworks (LangChain, OpenAI SDK, etc.) using the same validation capabilities.

Testing

• Tests added to the respective file if code was changed
• New code has 100% coverage if code as added
• Ensure existing tests and github automation passes (a maintainer will kick off the github automation when the rest of the PR is populated)

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