Enable RuboCop Minitest and RuboCop Rake for development

Based on the following information, `rubocop-minitest` and `rubocop-rake` have been added
for development. Autocorrection has been applied for offenses detected by `rubocop-minitest`.

```console
$ bundle exec rake
(snip)

Running RuboCop...
Inspecting 40 files
........................................

40 files inspected, no offenses detected

Tip: Based on detected gems, the following RuboCop extension libraries might be helpful:
* rubocop-minitest (https://rubygems.org/gems/rubocop-minitest)
* rubocop-rake (https://rubygems.org/gems/rubocop-rake)

You can opt out of this message by adding the following to your config
(see https://docs.rubocop.org/rubocop/extensions.html#extension-suggestions for more options):
AllCops:
    SuggestExtensions: false
```

If preserving the existing behavior is preferred, it is also possible to set `SuggestExtensions: false`.

test/model_context_protocol/transports/stdio_transport_test.rb

@@ -46,7 +46,7 @@ module ModelContextProtocol
           response = JSON.parse(output.string, symbolize_names: true)
           assert_equal("2.0", response[:jsonrpc])
           assert_equal("123", response[:id])
-          assert_equal({}, response[:result])
+          assert_empty(response[:result])
           refute(@transport.instance_variable_get(:@open))
         ensure
           $stdin = original_stdin