Add Rails request specs with ActiveRecord and fixtures

[Nitemaeric]

Summary

• Replace the standalone integration dummy (plain Ruby objects with hardcoded data) with a proper Rails dummy app backed by ActiveRecord and an in-memory SQLite database
• Test data managed through Rails fixtures instead of hardcoded class methods
• Fix Rendering#render to handle Rails' send_data, which passes a positional Hash to render rather than keyword arguments
• Add request specs covering serialization, namespace lookup, association rendering, cross-namespace serializers, and send_data compatibility

Test plan

• All 36 specs pass (bundle exec rspec)
• 99.22% line coverage
• No rubocop offenses
• CI passes across Ruby 3.2, 3.3, 3.4, and ruby-head

🤖 Generated with Claude Code