Add logging support

A server can send structured logging messages to the client.
https://modelcontextprotocol.io/specification/2025-06-18/server/utilities/logging#logging

Logging was specified in the 2024-11-05 specification, but since it was not supported in ruby-sdk, I implemented it.
https://modelcontextprotocol.io/specification/2024-11-05/server/utilities/logging

I also made it possible to output a simple notification message in the examples.

test/mcp/logging_message_notification_test.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module MCP
+  class LoggingMessageNotificationTest < ActiveSupport::TestCase
+    test "valid_level? returns true for valid levels" do
+      LoggingMessageNotification::LOG_LEVEL_SEVERITY.keys.each do |level|
+        logging_message_notification = LoggingMessageNotification.new(level: level)
+        assert logging_message_notification.valid_level?, "#{level} should be valid"
+      end
+    end
+
+    test "valid_level? returns false for invalid levels" do
+      invalid_levels = ["invalid", 1, "", nil, :fatal]
+      invalid_levels.each do |level|
+        logging_message_notification = LoggingMessageNotification.new(level: level)
+        refute logging_message_notification.valid_level?, "#{level} should be invalid"
+      end
+    end
+
+    test "should_notify? returns true when notification level is higher priority than threshold level or equals to it" do
+      logging_message_notification = LoggingMessageNotification.new(level: "warning")
+      assert logging_message_notification.should_notify?("warning")
+      assert logging_message_notification.should_notify?("error")
+      assert logging_message_notification.should_notify?("critical")
+      assert logging_message_notification.should_notify?("alert")
+      assert logging_message_notification.should_notify?("emergency")
+    end
+
+    test "should_notify? returns false when notification level is lower priority than threshold level" do
+      logging_message_notification = LoggingMessageNotification.new(level: "warning")
+      refute logging_message_notification.should_notify?("notice")
+      refute logging_message_notification.should_notify?("info")
+      refute logging_message_notification.should_notify?("debug")
+    end
+
+    test "should_notify? returns false for invalid log levels" do
+      logging_message_notification = LoggingMessageNotification.new(level: "warning")
+      refute logging_message_notification.should_notify?("invalid")
+      refute logging_message_notification.should_notify?(1)
+      refute logging_message_notification.should_notify?("")
+      refute logging_message_notification.should_notify?(nil)
+      refute logging_message_notification.should_notify?(:fatal)
+    end
+  end
+end