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)

test/mcp/server_test.rb

@@ -164,7 +164,7 @@ module MCP
       assert_kind_of Array, result[:tools]
       assert_equal "test_tool", result[:tools][0][:name]
       assert_equal "Test tool", result[:tools][0][:description]
-      assert_empty(result[:tools][0][:inputSchema])
+      assert_equal({ type: "object" }, result[:tools][0][:inputSchema])
       assert_instrumentation_data({ method: "tools/list" })
     end