Fix CSS plugin filtering and add cache immutability docs

CSS Plugin Filtering Improvements:
1. Added CssExtractRspackPlugin to plugin filter list (line 95)
   - Ensures Rspack's CSS plugin is properly removed from SSR bundle
   - Previously only filtered MiniCssExtractPlugin (Webpack)

2. Enhanced loader filtering with explicit Rspack check (lines 114-119)
   - Added regex match for CssExtractRspackPlugin
   - Ensures both Webpack and Rspack CSS loaders are filtered

Cache Immutability Documentation:
- Added detailed comment explaining Shakapacker config immutability
- Clarifies that config is loaded once at Node process start
- Documents that cache is safe because config cannot change at runtime

These changes address potential edge cases when using Rspack for SSR.

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

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

Author: justin808

config/webpack/bundlerUtils.js

@@ -10,7 +10,10 @@ const { config } = require('shakapacker');
 
 const VALID_BUNDLERS = ['webpack', 'rspack'];
 
-// Cache for bundler module (config is read at startup and cannot change without restart)
+// Cache for bundler module
+// IMPORTANT: Shakapacker config is immutable at runtime - it's loaded once when the
+// Node process starts. Changing shakapacker.yml requires restarting the server.
+// This cache is safe because config.assets_bundler cannot change during execution.
 let _cachedBundler = null;
 let _cachedBundlerType = null;
 

config/webpack/serverWebpackConfig.js

@@ -87,11 +87,12 @@ const configureServer = () => {
   };
 
   // Don't hash the server bundle b/c would conflict with the client manifest
-  // And no need for the MiniCssExtractPlugin
+  // And no need for CSS extraction plugins (MiniCssExtractPlugin or CssExtractRspackPlugin)
   serverWebpackConfig.plugins = serverWebpackConfig.plugins.filter(
     (plugin) =>
       plugin.constructor.name !== 'WebpackAssetsManifest' &&
       plugin.constructor.name !== 'MiniCssExtractPlugin' &&
+      plugin.constructor.name !== 'CssExtractRspackPlugin' &&
       plugin.constructor.name !== 'ForkTsCheckerWebpackPlugin',
   );
 
@@ -110,7 +111,12 @@ const configureServer = () => {
         } else if (typeof item.loader === 'string') {
           testValue = item.loader;
         }
-        return !(testValue?.match(/mini-css-extract-plugin/) || testValue?.includes('cssExtractLoader') || testValue === 'style-loader');
+        return !(
+          testValue?.match(/mini-css-extract-plugin/) ||
+          testValue?.match(/CssExtractRspackPlugin/) ||
+          testValue?.includes('cssExtractLoader') ||
+          testValue === 'style-loader'
+        );
       });
       const cssLoader = rule.use.find((item) => {
         let testValue;