This PR fixes incorrect capabilities handling for `completion/complete`.
As specified, the capability of `completion/complete` is `completions`, not `prompts`:
https://modelcontextprotocol.io/specification/2025-03-26/server/utilities/completion#capabilities
Note that in the Ruby SDK, `completion/complete`, like `logging/setLevel`, is not yet implemented.
Therefore, an empty implementation has been added to server.rb.