Allow server to return the negotiated version

[atesgoral]

https://modelcontextprotocol.io/specification/2025-03-26/basic/lifecycle

If the server supports the requested protocol version, it MUST respond with the same version. Otherwise, the server MUST respond with another protocol version it supports. This SHOULD be the latest version supported by the server.

However the gem currently responds with the configured/default protocol version instead of the negotiated version.

@koic What do you think about making the server dynamically return the version asked by the client (as long as we support it)?

[koic]

Good point. It looks like we can look into how other official SDK implementations handle this.