Add comprehensive coverage measurement with SimpleCov

[takaokouji]

Enhance test coverage measurement capabilities:

Changes:

• Configure SimpleCov with detailed settings in test_helper.rb
  • Add filters to exclude test/, config/, vendor/ directories
  • Add coverage groups (Controllers, Resources, Serializers, etc.)
  • Enable branch coverage tracking (Ruby 2.5+)
  • Configure multi-format output (HTML + Console)
• Clean up .gitignore (remove duplicate coverage entry)
• Update .dockerignore to exclude coverage artifacts
• Add coverage instructions to CLAUDE.md (local & Docker)

Test results with Rails 6.1.7.10:

• Line Coverage: 92.04% (3491/3793 lines)
• Branch Coverage: 85.13% (853/1002 branches)
• All 674 tests passing

Usage:
COVERAGE=true bundle exec rake test docker-compose run -e COVERAGE=true rails-6.1

🤖 Generated with Claude Code

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• I've checked to ensure there aren't other open Pull Requests for the same update/change.
• I've submitted a ticket for my issue if one did not already exist.
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New Feature Submissions:

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Bug fixes and Changes to Core Features:

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• I've provided test(s) that fails without the change.

Test Plan:

Reviewer Checklist:

• Maintains compliance with JSON:API
• Adequate test coverage exists to prevent regressions