test: add Ruby x Rails test matrix via Appraisal + GitHub Actions

[adi-herwana-nus]

• Appraisals + gemfiles/ for Rails 7.2, 8.0, 8.1
• .github/workflows/ruby.yml matrix: Ruby 3.1-3.4 x Rails 7.2/8.0/8.1 (Ruby 3.1 excluded on Rails 8.x, which requires Ruby >= 3.2)
• gemspec: add appraisal; swap unmaintained coveralls -> coveralls_reborn (the old gem pins an ancient simplecov and breaks resolution on newer Ruby)
• Only rails (activesupport + railties) is pinned per gemfile; no DB, so no rspec-rails/sqlite unlike userstamp
• Fix stale Python 3.14 spec: dependency is 'formulas[excel]', not 'formulas'
• test: assert docker_image returns nil when unset

Note: the matrix exercises Language as a PORO. In the main app Language is
an ActiveRecord model (see lib/extensions/polyglot_with_database there), so
a green matrix here does not cover the production path -- run the main app's
polyglot specs when validating a Rails upgrade.

An unset docker_image now intentionally returns nil (marking a language as
not runnable, e.g. a deprecated one) rather than deriving a name from the
class. Update the specs to the new contract: drop the obsolete
auto-generation cases and assert nil for concrete languages declared without
an explicit docker_image. The auto-generation method in ClassMethods is left
in place for now.