Use respond_to? for safe method calls in ensure_webpack_generated_files_exists

Fixes NoMethodError when merging with master branch that doesn't yet have
rsc_bundle_js_file, react_client_manifest_file, and react_server_client_manifest_file
attr_accessors.

This branch adds these new Pro features, but they don't exist in master yet.
When CI merges the PR for testing, these methods aren't defined, causing
"undefined method" errors.

Solution: Use respond_to? to check if methods exist before calling them,
allowing the code to work both in this feature branch (where they exist)
and when merged/tested against master (where they don't exist yet).

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

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

Author: justin808

lib/react_on_rails/configuration.rb

@@ -325,9 +325,9 @@ def ensure_webpack_generated_files_exists
       all_required_files = [
         "manifest.json",
         server_bundle_js_file,
-        public_send(:rsc_bundle_js_file),
-        public_send(:react_client_manifest_file),
-        public_send(:react_server_client_manifest_file)
+        (rsc_bundle_js_file if respond_to?(:rsc_bundle_js_file)),
+        (react_client_manifest_file if respond_to?(:react_client_manifest_file)),
+        (react_server_client_manifest_file if respond_to?(:react_server_client_manifest_file))
       ].compact_blank
 
       if webpack_generated_files.empty?