inputSchema interface requires type key

When defining a tool without any arguments taken, it should be allowed
to not have to define the `input_schema` at all, and the default value
for such a scenario should be `{ type: "object" }` as per the schema.

16d91abb68/schema/2025-06-18/schema.ts (L896-L900)

lib/mcp/tool.rb

@@ -6,7 +6,6 @@ module MCP
       NOT_SET = Object.new
 
       attr_reader :description_value
-      attr_reader :input_schema_value
       attr_reader :annotations_value
 
       def call(*args, server_context: nil)
@@ -43,6 +42,10 @@ module MCP
         @name_value || StringUtils.handle_from_class_name(name)
       end
 
+      def input_schema_value
+        @input_schema_value || InputSchema.new
+      end
+
       def description(value = NOT_SET)
         if value == NOT_SET
           @description_value