Enable RuboCop Minitest and RuboCop Rake for development

[koic]

Motivation and Context

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.

$ 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

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

This is a proposal intended to illustrate the code changes that would result from enabling the cop.
If preserving the existing behavior is preferred, it is also possible to set SuggestExtensions: false instead.