Fix RuboCop Performance/ConstantRegexp violation

Add /o option to regexp to optimize compilation performance.

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

Co-Authored-By: Claude <[email protected]>

Author: justin808

spec/react_on_rails/utils_spec.rb

@@ -465,7 +465,7 @@ def mock_dev_server_running
             s = { a: "1234567890" }
             result = described_class.smart_trim(s, 9)
             # Ruby 3.4+ changed hash syntax, so be flexible about the format
-            expect(result).to match(/\{(:a=|a: ")#{Regexp.escape(Utils::TRUNCATION_FILLER)}90"\}/)
+            expect(result).to match(/\{(:a=|a: ")#{Regexp.escape(Utils::TRUNCATION_FILLER)}90"\}/o)
           end
         end
       end