chore: refresh dependencies (keep parallel on 1.x) + hermetic test env

[igmarin]

What

Two related bits of release hygiene:

Dependencies

• Keep parallel on ~> 1.26 (resolves 1.28.0). parallel 2.x requires Ruby ≥ 3.3, which would silently break this gem's declared required_ruby_version = '>= 3.1'. The code only uses the stable Parallel.map(…, in_threads:) API, so there's no reason to require 2.x. This keeps Ruby 3.1/3.2 installable.
• Patch-bump cgi (~> 0.5.2) and json (~> 2.20); dev-dep concurrent-ruby (>= 1.3.7).
• .gitignore local review-tool artifacts.

Tests

• test_helper strips inherited *_API_KEY env vars so the validate/provider-resolution tests are hermetic. Previously, a real OPENAI_API_KEY exported in the shell made 'missing credentials' assertions pass, so rake test went red locally while CI (no keys) stayed green.

Verification

• rake test → 888 runs, 0 failures/errors (parallel 1.28.0).
• rubocop clean.
• parallel 1.28.0 requires Ruby ≥ 2.7, consistent with >= 3.1.

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR updates dependency version constraints in the Gemfile and gemspec (concurrent-ruby, cgi, json, parallel), adds new .gitignore patterns for rs-guard runtime artifacts, and modifies test_helper.rb to delete ENV variables matching _API_KEY before tests run to prevent leaked credentials from affecting test results.

Changes

Dependency and Configuration Updates

Layer / File(s)	Summary
Gem dependency version bumps Gemfile, ruby-skill-bench.gemspec	Updates concurrent-ruby to >= 1.3.7, cgi to ~> 0.5.2, json to ~> 2.20, and parallel to ~> 2.1.0.
Test environment hermeticity test/test_helper.rb	Deletes ENV keys matching _API_KEY before tests run to prevent leaked developer credentials from affecting assertions.
Gitignore additions .gitignore	Adds ignore patterns for .rs-guard/cache/, review-result.txt, and rs-guard-metrics.json.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

Poem

Hop along, the gems renewed,
No stray keys shall be construed,
.rs-guard's cache tucked out of sight,
A tidy burrow, clean and light,
Carrot-crunch, the build runs true! 🐇✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title captures the main themes of the PR: dependency refresh and a hermetic test environment.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

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