test: migrate the suite from RSpec to Minitest

[Halvanhelv]

Summary

Ports all six spec files to Minitest::Test with plain assertions, and drops the RSpec dependency.

What changed

• Style. Minitest::Test + assert_equal / refute_nil / assert_raises, no spec DSL. The context blocks that shared one it_behaves_like example are now tables driven through define_method, so every case still reports under its own name (test_tokenizes_a_notranslate_span_inside_another_span, …) rather than collapsing into a loop with one opaque failure.
• Doubles → fakes. The request test's RSpec doubles are replaced by two small classes. FakeApi records what it was asked to translate, so the test asserts on the call itself (assert_equal [[%w[One Two], :en, :ru, {}]], api.calls) instead of hiding the expectation inside a stub's argument matcher. FakeCacheStore always misses, which is what the specs did — cache_response was declared but never overridden.
• Plumbing. rspec → minitest ~> 6.0 in the gemspec, RSpec::Core::RakeTask → Rake::TestTask, both workflows run bundle exec rake test. .rspec, spec/ and the .rspec_status ignore rule are gone.
• Warnings. The test task sets t.warning = false. Rake::TestTask turns on -w by default, and unicode_utils and punkt-segmenter emit enough warnings to bury anything of ours.

Notes

• Coverage is unchanged: 267/282 lines, same 28 cases.
• spacing_spec.rb had a vestigial 4th element in each of its data rows that the destructuring ignored; the two cases are now written out explicitly.
• No new coverage was added — this is a port, not a test-writing pass. The cache-hit path is still untested, same as before.

Test plan

• bundle exec rake test — 28 runs, 34 assertions, 0 failures, on both Ruby 4.0.5 and 3.2.4.
• bundle exec rubocop — 0 offenses.
• Sanity-checked that the assertions bite: breaking Spacing.restore to drop the padding turns 3 tests red, and they go green again when it is restored.

https://claude.ai/code/session_01Pda49PcgziFVnibkKKjXRE