Add Ruby 4.0 compatibility

[nesquena]

Summary

Fixes test suite to pass on Ruby 4.0 (and continues to pass on Ruby 3.x). Ruby 4.0 introduced several breaking changes that affected padrino-framework:

Changes

Frozen string fixes (padrino-core/lib/padrino-core/application/routing.rb)

• Symbol#to_s now returns a frozen string in Ruby 4.0. Route paths derived from symbols (e.g. get :show) were being mutated in-place with gsub!, <<, etc., causing FrozenError. Fixed by calling .dup on the result of .to_s and on frozen string paths before mutation.
• String literals are also frozen by default in Ruby 4.0. Changed new_url = '' to String.new in rebase_url.

Removed default gems

• ostruct: Removed from Ruby 4.0 stdlib. Added as a runtime dependency in padrino-core.gemspec since padrino-core/configuration.rb requires it.
• logger: Removed from Ruby 4.0 stdlib. Added to Gemfile (needed by rack-protection).

Replaced oga with nokogiri (padrino/test/padrino/test-methods.rb)

• oga depends on ruby-ll, which has a C extension that fails to compile on Ruby 4.0. Replaced with nokogiri which is actively maintained and Ruby 4.0 compatible. Only used in test assertions.

Minitest mock extraction (padrino-core/test/helper.rb, padrino-cache/test/helper.rb)

• Object#stub was extracted from minitest 6.0 into the separate minitest-mock gem. Added the gem and require 'minitest/mock' to test helpers that use .stub.

Logger encoding fix (padrino-core/lib/padrino-core/logger.rb, padrino-core/test/test_logger.rb)

• Fixed encoding errors when writing binary/mixed-encoding data to the logger on Ruby 4.0, where StringIO defaults to US-ASCII. Binary data is now encoded to UTF-8 with replacement before writing.

Test Results (Ruby 4.0.2)

All 9 sub-gems pass: 1,441 runs, 5,465 assertions, 0 failures, 0 errors.

[nesquena]

@achiurizo Can you review this pull request? I'm thinking of merging this in and then at some point releasing 0.16.2. Should allow the full Padrino test suite to be green across all Ruby versions from 2.7 to Ruby 4. Since this only affects test files, I'll wait to release another version to rubygems. But this fixes the build matrix

[achiurizo]

LGTM — nice work getting Ruby 4.0 compat in. Left a few minor comments.

[achiurizo]

This changes behavior on Ruby 3.x too — binary/non-UTF-8 data that previously wrote through untouched will now get replacement characters when @sanitize_encoding isn't set.

Consider scoping to the log stream's encoding so it only kicks in when the write would actually fail:

Suggested change

	else
	line.encode!('UTF-8', invalid: :replace, undef: :replace) unless line.encoding == Encoding::UTF_8
	line.encode!(@log.external_encoding || Encoding::UTF_8, invalid: :replace, undef: :replace) unless line.encoding == Encoding::UTF_8

[nesquena]

Thanks for the review Arthur! Applied all three suggestions, with one caveat:

Logger encoding — good catch on the behavior change for 3.x. Now uses @log.external_encoding || Encoding::UTF_8 so it only encodes when the write would actually fail against the stream's encoding.

minitest-mock — I tried removing the Ruby version guard, but minitest-mock resolves to 5.27.0, which declares required_ruby_version >= 3.1. Bundler fails at resolution time on Ruby 2.7/3.0 before the begin/rescue LoadError in the test helpers ever gets a chance to run. Kept the guard but adjusted it to if RUBY_VERSION >= '3.1' to match the gem's own requirement.

ostruct — removed the redundant Gemfile entry since it's already in the gemspec.

Pushed the updates, tests still green across all sub-gems. Going to merge as soon as all the tests pass green.