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/input_schema.rb

@@ -15,8 +15,13 @@ module MCP
         validate_schema!
       end
 
+      def ==(other)
+        other.is_a?(InputSchema) && properties == other.properties && required == other.required
+      end
+
       def to_h
-        { type: "object", properties: }.tap do |hsh|
+        { type: "object" }.tap do |hsh|
+          hsh[:properties] = properties if properties.any?
           hsh[:required] = required if required.any?
         end
       end