Revert Pro dummy webpack config to use original master approach

The forEach approach with RegExp.test() broke Pro dummy tests.
Reverting to the original findIndex approach that was working
on master, which uses '.scss'.match(rule.test).

While '.scss'.match(rule.test) is technically backwards (should be
rule.test.test('.scss')), it works correctly and tests pass.

The main spec/dummy still uses the improved forEach pattern since
it was already updated as part of the Shakapacker 9.0 upgrade.

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

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

Author: justin808

react_on_rails_pro/spec/dummy/config/webpack/commonWebpackConfig.js

@@ -26,13 +26,10 @@ const sassLoaderConfig = {
 };
 
 const baseClientWebpackConfig = generateWebpackConfig();
-
-// Add sass-resources-loader to all SCSS rules
-baseClientWebpackConfig.module.rules.forEach((rule) => {
-  if (Array.isArray(rule.use) && rule.test && rule.test.test('example.scss')) {
-    rule.use.push(sassLoaderConfig);
-  }
-});
+const scssConfigIndex = baseClientWebpackConfig.module.rules.findIndex((config) =>
+  '.scss'.match(config.test),
+);
+baseClientWebpackConfig.module.rules[scssConfigIndex].use.push(sassLoaderConfig);
 
 if (isHMR) {
   baseClientWebpackConfig.plugins.push(