VoyageConfigKeys

Environment-variable names recognised by llm4s-voyage.

These were ConfigKeys.VOYAGE_* in llm4s-core until Voyage moved to its own module (#1132): a key belongs with the code that reads it, so that ConfigKeys does not name variables for a provider that may not be on the classpath. They are bound in this module's reference.conf under llm4s.embeddings.voyage.