Integrate ESLint rule and improve implementation

- Integrate no-use-client-in-server-files rule into eslint.config.ts
- Fix regex to only match at file start (remove multiline flag)
- Use template literal for cleaner error message
- Remove console.log from test file for cleaner CI output

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

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

Author: justin808

eslint-rules/no-use-client-in-server-files.cjs

@@ -23,10 +23,7 @@ module.exports = {
       url: 'https://github.com/shakacode/react_on_rails/pull/1896',
     },
     messages: {
-      useClientInServerFile:
-        "Files with '.server.tsx' extension should not have 'use client' directive. " +
-        'Server files are for React Server Components and should not use client-only APIs. ' +
-        'If this component needs client-side features, rename it to .client.tsx or .tsx instead.',
+      useClientInServerFile: `Files with '.server.tsx' extension should not have 'use client' directive. Server files are for React Server Components and should not use client-only APIs. If this component needs client-side features, rename it to .client.tsx or .tsx instead.`,
     },
     schema: [],
     fixable: 'code',
@@ -45,9 +42,10 @@ module.exports = {
         const sourceCode = context.sourceCode || context.getSourceCode();
         const text = sourceCode.getText();
 
-        // Check for 'use client' directive at the start of the file
+        // Check for 'use client' directive at the start of the file (not multiline)
         // Handle both single and double quotes, with or without semicolon
-        const useClientPattern = /^\s*['"]use client['"];?\s*$/m;
+        // Only matches at the very beginning of the file (not anywhere on its own line)
+        const useClientPattern = /^\s*['"]use client['"];?\s*\n?/;
         const match = text.match(useClientPattern);
 
         if (match) {

eslint-rules/no-use-client-in-server-files.test.cjs

@@ -157,5 +157,3 @@ import React from 'react';
     },
   ],
 });
-
-console.log('All tests passed!');

eslint.config.ts

@@ -8,6 +8,7 @@ import tsEslint from 'typescript-eslint';
 import { includeIgnoreFile } from '@eslint/compat';
 import js from '@eslint/js';
 import { FlatCompat } from '@eslint/eslintrc';
+import noUseClientInServerFiles from './eslint-rules/no-use-client-in-server-files.cjs';
 
 const compat = new FlatCompat({
   baseDirectory: __dirname,
@@ -165,6 +166,19 @@ const config = tsEslint.config([
       'import/named': 'off',
     },
   },
+  {
+    files: ['**/*.server.ts', '**/*.server.tsx'],
+    plugins: {
+      'react-on-rails': {
+        rules: {
+          'no-use-client-in-server-files': noUseClientInServerFiles,
+        },
+      },
+    },
+    rules: {
+      'react-on-rails/no-use-client-in-server-files': 'error',
+    },
+  },
   {
     files: ['lib/generators/react_on_rails/templates/**/*'],
     rules: {