Fix file existence checks in adapt_for_older_shakapacker_generator

Fixes CI failures where generator tests fail due to missing files:
- Add File.exist? checks before calling gsub_file on files that may not exist
- Add File.exist? check before renaming shakapacker.yml to webpacker.yml
- Add File.exist? check before modifying commonWebpackConfig.js

The adapt_for_older_shakapacker generator was failing in test environments
where config/shakapacker.yml and other expected files don't exist, causing
"No such file or directory" errors during RSpec generator tests.

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

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

Author: justin808

lib/generators/react_on_rails/adapt_for_older_shakapacker_generator.rb

@@ -19,24 +19,28 @@ def change_spelling_to_webpacker
           config/shakapacker.yml
           config/initializers/react_on_rails.rb
         ]
-        files.each { |file| gsub_file(file, "shakapacker", "webpacker") }
+        files.each { |file| gsub_file(file, "shakapacker", "webpacker") if File.exist?(file) }
       end
 
       def rename_config_file
-        puts "Rename to config/webpacker.yml"
-        puts "Renaming shakapacker.yml into webpacker.yml"
-        FileUtils.mv("config/shakapacker.yml", "config/webpacker.yml")
+        if File.exist?("config/shakapacker.yml")
+          puts "Rename to config/webpacker.yml"
+          puts "Renaming shakapacker.yml into webpacker.yml"
+          FileUtils.mv("config/shakapacker.yml", "config/webpacker.yml")
+        end
       end
 
       def modify_requiring_webpack_config_in_js
-        puts "Update commonWebpackConfig.js to follow the Shakapacker v6 interface"
         file = "config/webpack/commonWebpackConfig.js"
-        gsub_file(file, "const baseClientWebpackConfig = generateWebpackConfig();\n\n", "")
-        gsub_file(
-          file,
-          "const { generateWebpackConfig, merge } = require('shakapacker');",
-          "const { webpackConfig: baseClientWebpackConfig, merge } = require('shakapacker');"
-        )
+        if File.exist?(file)
+          puts "Update commonWebpackConfig.js to follow the Shakapacker v6 interface"
+          gsub_file(file, "const baseClientWebpackConfig = generateWebpackConfig();\n\n", "")
+          gsub_file(
+            file,
+            "const { generateWebpackConfig, merge } = require('shakapacker');",
+            "const { webpackConfig: baseClientWebpackConfig, merge } = require('shakapacker');"
+          )
+        end
       end
     end
   end