test: enforce 95% coverage with SimpleCov and close all spec gaps

[nejdetkadir]

Summary

• Adds SimpleCov with an enforced 95% line-coverage minimum on full-suite runs (bundle exec rake) and CI; the suite now sits at 100% line and 100% branch coverage (282 examples).
• Closes every documented test gap: real monad-contract service specs (KI-12), install generator specs (KI-13), non-default configuration specs (KI-14), plus token model and controller-helpers specs.
• Removes the dead TokensService::Create#authenticate_service method (KI-5) in a separate commit.

Changes

• Coverage tooling: simplecov ~> 0.22 in the Gemfile (compatible with the Ruby 2.7 CI floor); SimpleCov.start at the top of spec/spec_helper.rb before the gem loads; minimum enforced only when CI or ENFORCE_COVERAGE is set — a new enforce_coverage Rakefile prerequisite sets it for rake, so single-file rspec runs stay unaffected. version.rb is filtered (loaded by the gemspec before coverage can start).
• Service specs (spec/services/**): replaced the six inheritance-only placeholders with Success/Failure contract specs, including branches unreachable via HTTP (:invalid_resource_owner, :devise_api_token_create_error, :devise_api_token_revoke_error, sign-up transaction rollback).
• New specs: install_generator_spec.rb (migration template, locale copy, migration numbering), token_spec.rb (active?/inactive?, expires_in_infinite, generator collision retry, conditional validations), helpers_spec.rb (invalid-location ArgumentError, header-extraction rescue, unmemoized refresh-token lookup), configuration_overrides_spec.rb (disabled sign_up/refresh, extra_fields, revoke failure, :header/:params-only locations, with around/ensure config restore), admin_user_tokens_spec.rb, customized_path_names_routes_spec.rb.
• Dummy app: new bare AdminUser model (database_authenticatable, registerable, validatable, :api only) with table, migration, factory and devise_for :admin_users routes, so every "optional Devise module not enabled" branch is exercised end-to-end; users.name column added for extra_fields coverage.
• Response/config spec extensions: locked/unconfirmed/bare-model ErrorResponse variants (incl. unlock_at), TokenResponse refresh-disabled/extra-fields/non-confirmable bodies, Configuration overrides on fresh instances.
• Source: dead method removed from TokensService::Create; one pre-existing over-length line in token_response.rb reformatted for RuboCop (no behavior change).
• Docs (contractual): testing.md coverage section + fully-green coverage map, known-issues.md (KI-5/12/13 resolved, KI-14 mostly resolved), development.md, services.md.

Architecture Impact

• No production behavior changes. New dummy-app migrations (admin_users table, users.name) affect the test schema only — the gem's generator template is untouched.
• Gemfile.lock: simplecov + deps, arm64-darwin-25 platform entry, and recording the gem's own 0.1.3 → 0.2.0 drift.

Test Plan

• bundle exec rake green (282 examples, 0 failures; RuboCop 0 offenses)
• Line coverage 100% (441/441), branch coverage 100% (112/112)
• Enforcement verified: partial run under CI=true fails with the below-minimum error
• Suite run twice with --order random — no config leakage between examples

Related

• Resolves KI-5, KI-12, KI-13 and most of KI-14 from docs/analysis/known-issues.md

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