fix: resolve Ruby 4.0 compatibility issues in CI

- Add minitest-mock gem dependency (extracted from minitest in Ruby 4.0)
- Fix StandardRB indentation offenses in chunky_png_driver.rb

Author: Twill

gems.rb

@@ -16,6 +16,7 @@
   gem "capybara", ">= 3.26"
   gem "mutex_m" # Needed for RubyMine debugging.  Try removing it.
   gem "minitest", "< 6", require: false
+  gem "minitest-mock", require: false
   gem "minitest-stub-const", require: false
   gem "simplecov", require: false
   gem "rspec", require: false