SEP-1330: Elicitation Enum Schema Improvements

Implement support for [SEP-1330](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1330), when it lands.

I actually think we were erroneously not supporting `enumNames` in schema validation, so this may be a no-op. We should consider supporting `enumNames` in a backward-compatible way.