Add spec coverage for Client::Helper, query_string scheme, and Errors classes

[iamibi]

Summary

Closes #397

Line coverage dropped from 88.32% to 69.47% between v1.1.7 and v1.1.8. lib/oauth/client/ is the weakest real directory (~66%), and these had no dedicated spec file:

• OAuth::Client::Helper (lib/oauth/client/helper.rb) — used by every HTTP adapter
• The query_string OAuth signing scheme in lib/oauth/client/net_http.rb
• OAuth::Error / OAuth::Unauthorized / OAuth::Problem (lib/oauth/errors/)

This PR adds specs only — no behavior changes.

What changed

• spec/oauth/client/helper_spec.rb (new) — covers #nonce, #timestamp, #oauth_parameters (incl. allow_empty_params variants), #token_request?, #hash_body, #amend_user_agent_header, #header, #parameters, #parameters_with_oauth
• spec/net/http_spec.rb — added coverage for the query_string scheme (fresh query and appending to an existing one) and the URI-derivation branches in #signature_base_string (https fallback, :request_endpoint + :site)
• spec/oauth/error_spec.rb, spec/oauth/unauthorized_spec.rb, spec/oauth/problem_spec.rb (new) — cover OAuth::Error, OAuth::Unauthorized#to_s/#request, OAuth::Problem#to_s/#problem/#params
  • Note: these live at spec/oauth/*_spec.rb rather than mirroring lib/oauth/errors/, since RSpec/SpecFilePathFormat derives the expected path from the OAuth:: constant name, not the lib/ directory
• CHANGELOG.md — [Unreleased] entry

Test plan

• bundle exec kettle-test — 189 examples, 0 failures, 5 pending (unchanged, pre-existing pending specs for optional gems not installed)
• bundle exec rake rubocop_gradual:check — clean, only the pre-existing tracked Lint/EmptyWhen violation remains
• bin/rake reek — no new smells introduced by the new/changed spec files
• Confirmed via direct (non-parallel) coverage runs that the previously-uncovered lines in helper.rb, net_http.rb, and errors/*.rb are now exercised

[iamibi]

Looked into the failing checks — none of them are caused by the 5 files changed in this PR:

• All per-Ruby-version Specs jobs are green (2.4–3.4, current, dep-heads, JRuby, TruffleRuby).
• Code Coverage / Default rake task (locked & unlocked deps) fail because line coverage is still under the 80% hard gate. This job has been failing on main since at least 2026-07-27, so it's a pre-existing gap, not a regression from this PR — this PR actually nudges coverage up slightly (68.88% → 70.72%).
• Specs *-head@head jobs are explicitly continue-on-error: true in heads.yml for bleeding-edge interpreter builds, by design.
• run (Auto Assign) needs repo-write permissions forks don't get — expected on external PRs.
• Style shows violations across ~45 unrelated files (Gemfile, README, oauth.gemspec, consumer.rb, etc.). style.yml runs against Appraisal.root.gemfile, whose lock file isn't committed to the repo, so it resolves rubocop-lts fresh on every run — a newer release there looks like the cause, and it would likely show up on a fresh push to main too.

Happy to help address any of these separately if useful, but wanted to flag that they're unrelated to the spec changes here.

[iamibi]

Correction to my comment above — one more job is unrelated to this PR too: Specs - Ruby ruby current under the Windows workflow fails with Exec format error trying to run kettle-test.sh (Ruby 4.0.6 on Windows). Same root cause as Style: gemfiles/current.gemfile.lock isn't committed either, so that job resolves kettle-test fresh on every run — it pulled kettle-test 2.0.22, which looks like it has a Windows-specific packaging issue unrelated to the spec changes here. Windows passed on main's last run (2026-09-07) for the same reason — a newer, unpinned dependency release is the likely trigger, not this PR.

[pboling]

This looks great. I'll fix those CI issues in separate PRs.