`pkl-server` should expose both a transport schema version and the version of Pkl itself

There should be a `GET_VERSION` and a `GET_SCHEMA_VERSION` command defined on `pkl-server`, so-as to get versions from the server itself (instead of running the CLI separately)

Start by locating the pkl-server command-handling entry point and the existing transport protocol definitions. Trace how server responses expose version information, then define and verify GET_VERSION and GET_SCHEMA_VERSION so clients can retrieve both values without invoking the CLI.

Written by the indexing model from the issue text.