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/server_test.rb

@@ -153,7 +153,7 @@ module ModelContextProtocol
       assert_kind_of Array, result[:tools]
       assert_equal "test_tool", result[:tools][0][:name]
       assert_equal "Test tool", result[:tools][0][:description]
-      assert_equal({}, result[:tools][0][:inputSchema])
+      assert_empty(result[:tools][0][:inputSchema])
       assert_instrumentation_data({ method: "tools/list" })
     end