Add optional context parameter to embedding-based grounding

[nicoloesch]

Summary

ground_term() gains an optional context: str | None parameter, folded into the text passed to the on-demand query embedding call, letting callers supply disambiguating context alongside the term being grounded. No change to existing callers that omit it.

• Closes Increase Context for Grounding in RAG #19

[nicoloesch]

This now depends on AustralianCancerDataNetwork/omop-emb#46 to pass so it can be included nicely.

[nicoloesch]

Closing in favor of absorbing this into #34

The on-demand embedding path this PR touches (KnowledgeGraph.emb, ground_term()) is being restructured in that same work. Merging this first would mean redoing it again immediately after, for a release nobody would pull in between.

The context parameter design here is unchanged and lands as part of that PR, with credit to this branch. Reopen if there's a reason to decouple it.