Add RAG groundedness evaluation assertion

[dimplebhardwaj2]

Hi! I was using ghostrun in one of my RAG projects and realized I needed a way to verify whether the generated answer was actually grounded in the retrieved context. I ended up implementing a solution on my own, and while exploring the repository I noticed ghostrun already provides semantic assertions like contains_intent, tone_is, and matches, but doesn't yet have a dedicated RAG grounding assertion.

I'd love to contribute this back to the project by adding something like:

ghostrun.expect(answer).is_grounded_in(context)

If this aligns with the project's direction, I'd be happy to work on it and open a PR.

[parthmax2]

Thanks for raising this. I think is_grounded_in(context) would be a valuable addition and aligns well with Ghostrun's assertion API.

Please go ahead and open a PR. Keep the API consistent with the existing assertions, and include tests and documentation. Happy to review it.