Fix incorrect capabilities handling for completion/complete

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.

lib/model_context_protocol/methods.rb

@@ -36,7 +36,7 @@ module ModelContextProtocol
 
     def ensure_capability!(method, capabilities)
       case method
-      when PROMPTS_GET, PROMPTS_LIST, COMPLETION_COMPLETE
+      when PROMPTS_GET, PROMPTS_LIST
         unless capabilities[:prompts]
           raise MissingRequiredCapabilityError.new(method, :prompts)
         end
@@ -56,6 +56,10 @@ module ModelContextProtocol
         unless capabilities[:sampling]
           raise MissingRequiredCapabilityError.new(method, :sampling)
         end
+      when COMPLETION_COMPLETE
+        unless capabilities[:completions]
+          raise MissingRequiredCapabilityError.new(method, :completions)
+        end
       when LOGGING_SET_LEVEL
         # Logging is unsupported by the Server
         unless capabilities[:logging]