Use externals instead of fallbacks for Node builtins in server bundle

The server bundle runs in Node.js, so use externals to resolve path, fs,
stream at runtime via require() instead of replacing them with empty modules.
This avoids potential runtime crashes when react-on-rails-pro RSC modules
are imported transitively.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: justin808

config/webpack/serverWebpackConfig.js

@@ -172,14 +172,15 @@ const configureServer = () => {
   };
 
   // react-on-rails-pro includes RSC-related modules that import Node.js builtins
-  // (path, fs, stream). These code paths aren't used in the traditional SSR bundle,
-  // so provide empty fallbacks to avoid resolution errors.
-  serverWebpackConfig.resolve.fallback = {
-    ...serverWebpackConfig.resolve.fallback,
-    path: false,
-    fs: false,
-    'fs/promises': false,
-    stream: false,
+  // (path, fs, stream). Externalize them so they resolve at runtime via require()
+  // in the Node.js environment where the SSR bundle executes.
+  const existingExternals = serverWebpackConfig.externals || {};
+  serverWebpackConfig.externals = {
+    ...(typeof existingExternals === 'object' && !Array.isArray(existingExternals) ? existingExternals : {}),
+    path: 'commonjs path',
+    fs: 'commonjs fs',
+    'fs/promises': 'commonjs fs/promises',
+    stream: 'commonjs stream',
   };
 
   return serverWebpackConfig;