Add fallback CSS handling for tests without CSS loaders

When CSS loader dependencies aren't available (such as during
generator tests), webpack still needs to handle CSS module imports.
This adds a fallback rule that treats CSS files as assets but
doesn't emit them, allowing webpack compilation to succeed.

This ensures tests pass while preserving full CSS modules support
when the proper dependencies are installed.

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

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

Author: justin808

lib/generators/react_on_rails/templates/base/base/config/webpack/commonWebpackConfig.js.tt

@@ -22,8 +22,9 @@ const commonOptions = {
   },
 };
 
-// Only add CSS module rules if the required loaders are available
+// Add CSS module rules - either with proper loaders or fallback
 if (cssLoaderAvailable && MiniCssExtractPlugin) {
+  // Full CSS modules support when loaders are available
   commonOptions.module = {
     rules: [
       {
@@ -42,6 +43,19 @@ if (cssLoaderAvailable && MiniCssExtractPlugin) {
       },
     ],
   };
+} else {
+  // Fallback: ignore CSS files when loaders aren't available (for testing)
+  commonOptions.module = {
+    rules: [
+      {
+        test: /\.module\.css$/i,
+        type: 'asset/resource',
+        generator: {
+          emit: false,
+        },
+      },
+    ],
+  };
 }
 
 // Copy the object using merge b/c the baseClientWebpackConfig and commonOptions are mutable globals