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]

lib/model_context_protocol/server.rb

@@ -60,6 +60,7 @@ module ModelContextProtocol
         # No op handlers for currently unsupported methods
         Methods::RESOURCES_SUBSCRIBE => ->(_) {},
         Methods::RESOURCES_UNSUBSCRIBE => ->(_) {},
+        Methods::COMPLETION_COMPLETE => ->(_) {},
         Methods::LOGGING_SET_LEVEL => ->(_) {},
       }
     end

test/model_context_protocol/methods_test.rb

@@ -19,6 +19,13 @@ module ModelContextProtocol
       assert_equal "Server does not support sampling (required for sampling/createMessage)", error.message
     end
 
+    test "ensure_capability! for completion/complete raises an error if completions capability is not present" do
+      error = assert_raises(Methods::MissingRequiredCapabilityError) do
+        Methods.ensure_capability!(Methods::COMPLETION_COMPLETE, {})
+      end
+      assert_equal "Server does not support completions (required for completion/complete)", error.message
+    end
+
     test "ensure_capability! for logging/setLevel raises an error if logging capability is not present" do
       error = assert_raises(Methods::MissingRequiredCapabilityError) do
         Methods.ensure_capability!(Methods::LOGGING_SET_LEVEL, {})