Fix require path in RBS runtime checking spec

Change require_relative from "../spec_helper" to "spec_helper" to match
the correct path structure. The spec_helper is located at
spec/react_on_rails/spec_helper.rb, not spec/spec_helper.rb.

This fixes the LoadError in CI minimum dependency tests.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

spec/react_on_rails/rbs_runtime_checking_spec.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require_relative "../spec_helper"
+require_relative "spec_helper"
 
 # This spec validates that RBS runtime type checking catches actual type violations
 # when enabled during test execution. These tests should only run when RBS is available.